0.1: Fundamental Skills in Algebra
Algebra is the branch of mathematics that studies operations with numbers—namely, by generalizing arithmetic by using symbols to represent numbers and relationships. It underpins much of calculus and beyond, so any calculus student needs to master its basic operations. So this section covers the following topics:
Solving Equations and Inequalities
In a mathematical equation, two quantities have the same value. For example, \(3 = 2 + 1\) is a valid equation because both sides have the same value. But some equations have some unknown quantity, such as the equation \(2x + 5 = 7.\) What does \(x\) equal? We can manipulate equations by adding, subtracting, multiplying, or dividing a number on both sides. On both sides of \(2x + 5 = 7,\) subtracting \(5\) and then dividing by \(2\) yields \(2x = 2\) and then \(x = 1.\) The following guidelines generalize the strategy for solving similar equations. In general, the objective is to perform operations to isolate the variable.
- Condense On each side, simplify the expression by combining like terms.
- Separate By adding or subtracting, gather all the variables on one side of the equation and all the constants on the other side.
- Isolate Divide both sides by the coefficient.
- Check Substitute your value of \(x\) back into the equation to verify whether it is correct.
- Condense On the left side, adding \(6\) and \(10\) gives \(16.\) On the right side, adding \(-8x\) to \(2x\) yields \(-6x.\) Thus, the condensed equation is \[3x + 16 = 7 - 6x \pd\]
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Separate
We want to place all the
\(x\)'s on the left and all the constants on the right. To clear the \(-6x\) on the right, we add \(6x\) to both sides; to clear the \(16\) on the left, we subtract \(16\) from both sides. Doing so yields \[9x = -9 \pd\] - Isolate The coefficient is the number by which \(x\) is multiplied—namely, \(9.\) To cancel out the \(9\) and therefore isolate \(x,\) we divide both sides by \(9\) to get \[x = \boxed{-1}\]
- Check Substituting \(x = -1\) back into the given equation, we see \[ \ba 3(-1) + 6 + 10 &= 7 - 8(-1) + 2(-1) \nl -3 + 6 + 10 &= 7 + 8 - 2 \nl 13 &\equalsCheck 13 \pd \ea \] This step verifies that \(x = -1\) is the correct solution.
- Condense To expand \(4(x + 5),\) we distribute the \(4\)—that is, multiply each term inside the parentheses by \(4.\) Then the equation becomes \[ \ba 4x + 20 - 2x &= 31 \nl 2x + 20 &= 31 \pd \ea \]
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Separate
We want all the
\(x\)'s to be on the left side and all the constants to be on the right side. Subtracting \(20\) from both sides yields \[2x = 11 \pd\] - Isolate To cancel out the coefficient of \(2,\) we divide both sides by \(2\) to attain \[x = \boxed{\tfrac{11}{2}}\] (It is also correct to write \(x = 5.5;\) in calculus we prefer to write fractions.)
- Check Substituting \(x = 11/2\) back into the given equation, we see \[ \ba 4 \par{\tfrac{11}{2} + 5} - 2 \par{\tfrac{11}{2}} &= 31 \nl (22 + 20) - 11 &= 31 \nl 31 &\equalsCheck 31 \pd \ea \] Hence, \(x = 11/2\) is the correct solution.
An inequality is a mathematical statement that compares two quantities; for example, \(5 \gt 4\) means \(5\) is greater than \(4.\) Likewise, the inequality \(x \leq 6\) means \(x\) can be any number that is less than or equal to \(6.\) We solve inequalities similarly to how we solve equations, as shown by the following examples.
- \(6x + 3 \gt 15\)
- \(-4x - 6 \leq 10\)
- \(\dfrac{1}{x} - 2 \gt 5 \sspace\) if \(\sspace x \gt 0\)
- On both sides, we subtract \(3\) and then divide by \(6 \col\) \[6x \gt 12 \implies \boxed{x \gt 2}\]
- On both sides, we add \(6\) and then divide by \(-4.\) But when we multiply or divide both sides by a negative number, the inequality sign is flipped. So we have \[-4x \leq 16 \implies \boxed{x \geq -4}\]
- Adding \(2\) to both sides gives \[\frac{1}{x} \gt 7 \pd\] We multiply both sides by \(x\) and then divide by \(7,\) as follows: \[1 \gt 7x \implies \tfrac{1}{7} \gt x \implies x \lt \tfrac{1}{7} \pd\] (Since \(x\) is given to be positive, we don't flip the inequality sign.) The problem gives \(x \gt 0,\) so we write the solution as \[\boxed{0 \lt x \lt \tfrac{1}{7}}\]
Recall that exponents represent repeated multiplication; for example, \(2^3\) \(= 2 \cdot 2 \cdot 2\) \(= 8.\) Let \(a\) and \(b\) be positive numbers, and let \(m\) and \(n\) be any numbers. Then the exponent laws are as follows: \begin{align} a^{m + n} &= a^m \, a^n \cma \label{eq:exp-law-add} \nl a^{m - n} &= \frac{a^m}{a^n} \cma \label{eq:exp-law-minus} \nl \par{a^{m}}^n &= a^{mn} \cma \label{eq:exp-law-mn} \nl \par{ab}^n &= a^n \, b^n \pd \label{eq:exp-law-a-b} \end{align} We won't prove these properties, but you can perform some testing to verify that they are true. (We will study these properties more in depth in Section 0.9.)
A polynomial is a sum or difference of variables with nonnegative integer (whole number) powers. A polynomial is classified by its highest-power term. For example, \(x^3 + 5x^2 - 8\) is a third-degree polynomial and \(x^6 + x - 2\) is a sixth-degree polynomial. A second-degree polynomial is called quadratic; we investigate quadratic expressions and functions more in Section 0.7. Polynomials are prevalent in calculus; we devote the remainder of this section to operations with polynomials.
- \((2x + 4)(x - 3)\)
- \((5x + 1)^2\)
- Breaking up the first set of parentheses, we get \[2x(x - 3) + 4(x - 3) \pd\] Then distributing and combining like terms produce \[ \ba (2x^2 - 6x) + (4x - 12) &= 2x^2 - 6x + 4x - 12 \nl &=\boxed{2x^2 - 2x - 12} \ea \]
- It is wrong to write \[\wrongMath{(5x + 1)^2 = (5x)^2 + 1^2 = 25x^2 + 1 \pd}\] Instead, note that \((5x + 1)^2\) \(= (5x + 1)(5x + 1).\) Breaking up the first set of parentheses, we have \[5x(5x + 1) + 1(5x + 1) \pd\] Then we distribute and combine like terms, as follows: \[ \ba (25x^2 + 5x) + (5x + 1) &= 25x^2 + 5x + 5x + 1 \nl &= \boxed{25x^2 + 10x + 1} \ea \]
Factoring Polynomials
Factoring is the process of decomposing a number or expression into the product of two or more terms (factors). For example, the number \(10\) is factored as \(2 \times 5.\) There are several techniques to factor polynomials. The most important one is by extracting the greatest common factor (GCF), the largest factor common to all terms in the expression. This process is the first step of factoring a polynomial.
- \(4x + 8\)
- \(9ab + 3b\)
- The greatest common factor of \(4x\) and \(8\) is \(4,\) which we extract to get \[\boxed{4(x + 2)}\]
- The greatest common factor of the coefficients \(9\) and \(3\) is \(3.\) Both terms also contain a \(b,\) so extracting \(3b\) gives \[\boxed{3b(3a + 1)}\]
The following formulas enable us to factor differences of squares, sums of cubes, and differences of cubes.
- \(x^2 - 81\)
- \(y^3 + 27\)
- Observe that \(81\) \(= 9^2.\) Using \(\eqref{eq:diff-sq}\) with \(a = x\) and \(b = 9,\) we get \[\boxed{(x + 9)(x - 9)}\]
- Note that \(27\) \(= 3^3.\) Using \(\eqref{eq:sum-cb}\) with \(a = y\) and \(b = 3\) yields \[\boxed{(y + 3)(y^2 - 3y + 9)}\]
Next we examine the strategy for factoring the family of quadratics \(x^2 + bx + c\) into two binomials. (A binomial is the sum or difference of two terms.) The objective is to find two numbers \(p\) and \(q\) whose product is \(c\) and sum is \(b;\) then \[x^2 + bx + c = (x + p)(x + q) \pd\] The following example demonstrates this procedure.
Let's now discuss factoring by grouping. Using this technique, we split a polynomial into pairs of terms, each of which we factor independently. Factoring by grouping works if each pair shares a common factor; then the polynomial's factored form contains the shared factor multiplied by the sum of the groups' GCFs, as shown by the following examples.
Factoring by grouping is useful for factoring quadratics in the form \(ax^2 + bx + c\) \((a \ne 0)\) when no GCF is present. The objective is to find two numbers \(p\) and \(q\) whose product is \(ac\) and sum is \(b,\) and then split \(bx\) into \(px + qx.\) The resulting expression can be factored by grouping, as shown by the next example.
Operations with Fractions
Fractions represent the division of two quantities. Recall that the top part of a fraction is the numerator and the bottom part is the denominator. Multiplying fractions is easy—we simply multiply across; for example, \[\tfrac{2}{3} \times \tfrac{5}{6} = \tfrac{10}{18} = \tfrac{5}{9} \pd\] (In the last step we divided the numerator and denominator each by \(2.\) Multiplying or dividing the numerator and denominator by the same number does not alter the fraction's value.) In dividing two fractions, we flip the second fraction and perform multiplication. As an example, to divide \(\tfrac{4}{5}\) by \(\tfrac{8}{7},\) we have \[\tfrac{4}{5} \times \tfrac{7}{8} = \tfrac{28}{40} = \tfrac{7}{10} \pd\] The fractions \(\tfrac{8}{7}\) and \(\tfrac{7}{8}\) are reciprocals of each other. The same logic with numerical fractions applies to fractions with algebraic expressions, as in the next examples.
To add or subtract two fractions with the same denominator, we simply add or subtract the numerators; a simple example is \(\tfrac{3}{5} + \tfrac{1}{5}\) \(= \tfrac{4}{5}.\) But if no common denominator is present, then we multiply both parts of one fraction (or both fractions) to force the existence of one. For example, \[\tfrac{1}{2} + \tfrac{1}{3} = \tfrac{3}{6} + \tfrac{2}{6} = \tfrac{5}{6} \pd\] The same idea applies to rational functions.
To simplify a quotient of polynomials that may share no common factors, we use polynomial division, which is analogous to standard long division for numbers. The following steps describe how to perform this operation.
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Write out the terms of \(P\) in order of descending powers.
To the left of the expression, draw a right parenthesis
\()\)
; above the expression, draw a horizontal line. To the left of the parenthesis, write out the terms of \(Q\) in order of descending powers. - Divide the first term of \(P\) by the first term of \(Q;\) place that answer above the horizontal line. Multiply \(Q\) by that answer and write the result in a new row at the bottom.
- Subtract the result of Step 2 from \(P\) to generate a new polynomial.
- Divide the first term of the new polynomial by the first term of \(Q.\) Multiply \(Q\) by that result and subtract it from the new polynomial. Repeat this process until the resulting polynomial has a degree smaller than the degree of \(Q.\)
\()\)and draw a horizontal line above the first polynomial. Next we divide the first terms—that is, \(2x^3\) divided by \(x\)—to get \(2x^2,\) which we write at the top. Then \(2x^2 (x - 1)\) \(= 2x^3 - 2x^2,\) which we subtract from \(2x^3 + x^2\) \(+ \, x - 4\) to generate a new polynomial, \(3x^2 + x - 4.\) We repeat this process, as shown by the following setup. \[ \require{enclose} \begin{array}{rll} 2x^2 + 3x + 4 \\[-3pt] x - 1 \enclose{longdiv}{2x^3 + \phantom{2} x^2 + x - 4}\kern-.2ex \nl \begin{alignedat}{3} 2x^3 \hspace{0.32em} &- &&\, \, 2x^2 & \nl \hline & &&3x^2 &+ \phantom{3} x \phantom{-4} \nl & &&3x^2 &- 3x \phantom{+0} \nl \hline & && &\phantom{+} 4x - 4 \nl & && &\phantom{+} 4x - 4 \nl \hline & && & \hspace{0.4em} 0 \end{alignedat} \end{array} \] Thus, \[\frac{2x^3 + x^2 + x - 4}{x - 1} = \boxed{2x^2 + 3x + 4}\]
Solving Equations and Inequalities Algebra is the branch of mathematics concerned with generalizing operations with numbers. Solving equations and inequalities is the heart of algebra. The following steps enable us to solve an equation for some variable \(x \col\)
- Condense On each side, simplify the expression by combining like terms.
- Separate By adding or subtracting, gather all the variables on one side of the equation and all the constants on the other side.
- Isolate Divide both sides by the coefficient.
- Check Substitute your value of \(x\) back into the equation to verify whether it is correct.
A similar process enables us to solve inequalities for \(x.\) Remember that the inequality sign is flipped when both sides are multiplied or divided by a negative number. If \(a\) and \(b\) are positive numbers, and \(m\) and \(n\) are any numbers, then the exponent laws are as follows: \begin{align} a^{m + n} &= a^m \, a^n \cma \eqlabel{eq:exp-law-add} \nl a^{m - n} &= \frac{a^m}{a^n} \cma \eqlabel{eq:exp-law-minus} \nl \par{a^{m}}^n &= a^{mn} \cma \eqlabel{eq:exp-law-mn} \nl \par{ab}^n &= a^n \, b^n \pd \eqlabel{eq:exp-law-a-b} \end{align}
Factoring Polynomials Factoring is the process by which a number or expression is decomposed into the product of two or more terms (factors). A polynomial is a sum or difference of variables with nonnegative integer (whole number) powers; a polynomial is classified by its highest power. The first step to factoring a polynomial is to extract the greatest common factor (GCF), the largest number or expression that goes into each term in the expression. Subsequent methods of factoring are as follows.
- Factoring \(x^2 + bx + c\) By finding two numbers \(p\) and \(q\) whose product is \(c\) and sum is \(b,\) we attain \[x^2 + bx + c = (x + p)(x + q) \pd\]
- Factoring Squares and Cubes To factor two terms each raised to the power of \(2\) or \(3,\) one can use the following formulas: \begin{alignat}{2} a^2 - b^2 &= (a + b)(a - b) \comment{\text{Difference of Squares}} \eqlabel{eq:diff-sq} \nl a^3 + b^3 &= (a + b)(a^2 - ab + b^2) \comment{\text{Sum of Cubes}} \eqlabel{eq:sum-cb} \nl a^3 - b^3 &= (a - b)(a^2 + ab + b^2) \comment{\text{Difference of Cubes}} \eqlabel{eq:diff-cb} \end{alignat}
- Factoring by Grouping A polynomial can be split into pairs of terms, each of which is factored independently. If each pair shares a common factor, then the resulting polynomial is the product of two sets of parentheses: one containing the shared factor, and the other containing the sum of the groups' GCFs.
To factor \(ax^2 + bx + c\) \((a \ne 0),\) we find two numbers \(p\) and \(q\) whose product is \(ac\) and sum is \(b.\) Then we split \(bx\) into \(px + qx\) and factor the resulting polynomial by grouping.
Operations with Fractions The procedures for adding, subtracting, multiplying, and dividing numerical fractions are identical to those for fractions containing algebraic expressions. Multiplying fractions is the simplest operation—simply multiply the numerators together and multiply the denominators together. Dividing two fractions is equivalent to multiplying the first fraction by the reciprocal of the second fraction. To add or subtract two fractions, attain a common denominator; if one does not exist, then multiply the numerator and denominator of one fraction (or both fractions) to force the existence of a common denominator. Polynomial division enables us to decompose a quotient of two polynomials; it is analogous to long division for numbers. The steps for evaluating \(P/Q,\) where \(P\) and \(Q\) are polynomials such that the degree of \(Q\) is less than the degree of \(P,\) are as follows:
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Write out the terms of \(P\) in order of descending powers.
To the left of the expression, draw a right parenthesis
\()\)
; above the expression, draw a horizontal line. To the left of the parenthesis, write out the terms of \(Q\) in order of descending powers. - Divide the first term of \(P\) by the first term of \(Q;\) place that answer above the horizontal line. Multiply \(Q\) by that answer and write the result in a new row at the bottom.
- Subtract the result of Step 2 from \(P\) to generate a new polynomial.
- Divide the first term of the new polynomial by the first term of \(Q.\) Multiply \(Q\) by that result and subtract it from the new polynomial. Repeat this process until the resulting polynomial has a degree smaller than the degree of \(Q.\)