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Solve x squared plus 10x equal to 24 by completing the square.
Ax = 2 and x = 12
Bx = -2 and x = 12
Cx = 2 and x = -12
Dx = 5 and x = -5
Answer & Solution
Correct answer: C. x = 2 and x = -12
1. Completing the square adds the square of half the coefficient of x to both sides.
2. Half of 10 is 5, and 5 squared is 25.
3. Adding 25 to both sides gives x squared plus 10x plus 25 equal to 49.
4. The left side is now a perfect square, so it factors as the bracket (x plus 5) squared.
5. Take the square root of both sides, keeping both signs, so x plus 5 equals 7 or negative 7.
6. The first branch gives x equal to 2 and the second gives x equal to negative 12.
7. Checking, 2 squared plus 10 times 2 is 4 plus 20, which is 24.
8. Checking the other, negative 12 squared is 144 and 10 times negative 12 is negative 120, and 144 minus 120 is 24.
9. The pair 2 and 12 keeps the wrong sign on the second branch.
10. The pair 5 and negative 5 solves only the halved coefficient rather than the equation.
_Source: OpenStax Intermediate Algebra (CC BY 4.0), section 9.2 Solve Quadratic Equations by Completing the Square_