SOLUTION: x^2 - 12x =45 x^2 -12x +36=81 (need to know how my text book got +36 and 81) (x-6)^2 =81 x-6= +-9 x= +-9 +6 x=15 or -3

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equations Lessons  -> Quadratic Equation Lesson -> SOLUTION: x^2 - 12x =45 x^2 -12x +36=81 (need to know how my text book got +36 and 81) (x-6)^2 =81 x-6= +-9 x= +-9 +6 x=15 or -3       Log On


   

Question 184090: x^2 - 12x =45
x^2 -12x +36=81 (need to know how my text book got +36 and 81)
(x-6)^2 =81
x-6= +-9
x= +-9 +6
x=15 or -3
Found 2 solutions by Earlsdon, jim_thompson5910:
Answer by Earlsdon(6294) About Me  (Show Source):
You can put this solution on YOUR website!
I really can't anwer the question you raised but your solution is the correct one!
Remember, book-answers are not infallible!

Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
x%5E2+-+12x+=45 Start with the given equation


What the book is doing is completing the square on the left side (which will help isolate "x").


x%5E2+-+12x+%2B36=45%2B36 Take half of the "x" coefficient -12 to get -6. Square -6 to get 36. Add this value to both sides.


x%5E2+-+12x+%2B36=81 Combine like terms.


%28x-6%29%5E2=81 Factor the left side


x-6=0%2B-+sqrt%2881%29 Take the square root of both sides.


x-6=sqrt%2881%29 or x-6=-sqrt%2881%29 Break up the "plus/minus"


x-6=9 or x-6=-9 Take the square root of 81 to get 9


x=6%2B9 or x=6-9 Add 6 to both sides.


x=15 or x=-3 Combine like terms.


So the solutions are x=15 or x=-3