Solve x² + 6x − 7 = 0 by completing the square
Quadratic equation, worked out line by line the way a teacher would write it.
Answer
| Solutions | x = −7, x = 1 |
Step-by-step solution
8 steps-
1 Givenx^{2} + 6 x - 7 = 0
Solve for x.
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2 This is a quadratic in standard form ax² + bx + c = 0
Every quadratic equation can be arranged as ax² + bx + c = 0. Reading off a = 1, b = 6 and c = -7, signs included, shows which method fits: factoring, taking a square root or the quadratic formula.
a = 1,\quad b = 6,\quad c = -7 -
3 Add 7 to both sides
An equation stays true as long as you do the same thing to both sides, like adding the same weight to both pans of a balance. Adding or subtracting the same amount on both sides moves a term to the other side, where it appears with the opposite sign.
x^{2} + 6 x = 7 -
4 Add (b/2)² = 9 to both sides to complete the square
(x + h)² = x² + 2hx + h². With h = 3, half of the x coefficient, adding h² = 9 makes the left side a perfect square, (x + h)². The same amount is added on the right to keep the equation balanced; then a square root finishes the job.
x^{2} + 6 x + 9 = 7 + 9Half the x coefficient is 3; its square is 9.
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5 The left side is now a perfect square\left(x + 3\right)^{2} = 16
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6 Take the square root of both sides — remember both signs
A number and its negative give the same result when raised to an even power: 3² = 9 and (−3)² = 9. So if something squared equals k, that something is √k or −√k. Forgetting the minus sign loses a solution.
x + 3 = \pm 4 -
7 Subtract 3 from both sidesx = -7\quad \text{or} \quad x = 1
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8 Check\begin{aligned}x = -7:\quad 0 = 0\quad\checkmark\\ x = 1:\quad 0 = 0\quad\checkmark\end{aligned}
Substituting each solution back makes both sides equal.
Check by substitution
| x | Left side | Right side | |
|---|---|---|---|
| −7 | 0 | 0 | ✓ |
| 1 | 0 | 0 | ✓ |
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