Question

Solve x2 = 12x – 15 by completing the square. Which is the solution set of the equation?

Answer 1

Answer:

  {6-√21, 6+√21}

Step-by-step explanation:

It usually works well to put the x-terms on one side of the equal sign. Then add the square of half the x-term coefficient.

  x^2 -12x = -15 . . . . . . subtract 12x

The x-term coefficient is -12, so the square of half that is (-12/2)^2 = 36. Adding 36, we have ...

  x^2 -12x +36 = -15 +36

  (x -6)^2 = 21

  x -6 = ±√21 . . . . . take the square root

  x = 6 ±√21 . . . . . add 6

The solution set is ...

  {6-√21, 6+√21}