SOLUTION: My book says the answer is -5 and -3, where am I missing it? {{{x^2-8x-15=0}}} x={{{(-(-8)+- sqrt(-8^2-4*1*15))/(2*1)}}} x={{{8+- sqrt (64-60)/(2)}}} x={{{8+- sqrt (4)/(2)}}}

Algebra ->  Quadratic Equations and Parabolas -> SOLUTION: My book says the answer is -5 and -3, where am I missing it? {{{x^2-8x-15=0}}} x={{{(-(-8)+- sqrt(-8^2-4*1*15))/(2*1)}}} x={{{8+- sqrt (64-60)/(2)}}} x={{{8+- sqrt (4)/(2)}}}       Log On


   

Question 259973: My book says the answer is -5 and -3, where am I missing it?
x%5E2-8x-15=0
x=%28-%28-8%29%2B-+sqrt%28-8%5E2-4%2A1%2A15%29%29%2F%282%2A1%29
x=8%2B-+sqrt+%2864-60%29%2F%282%29
x=8%2B-+sqrt+%284%29%2F%282%29
x=%288%2B2%29%2F%282%29=+10%2F2+=+5
x=%288-2%29%2F%282%29=+6%2F2+=3
Answer by nerdybill(7384) About Me  (Show Source):
You can put this solution on YOUR website!
You are both wrong!
.
Your mistake:
x%5E2-8x-15=0
x=%28-%28-8%29%2B-+sqrt%28-8%5E2-4%2A1%2A15%29%29%2F%282%2A1%29 <--Here -- should be -15
.
Unless your original equation has a typo then the quadratic equation yields:
x = {9.568, -1.568}
Details of quadratic equation follows:
.
Solved by pluggable solver: SOLVE quadratic equation with variable
Quadratic equation ax%5E2%2Bbx%2Bc=0 (in our case 1x%5E2%2B-8x%2B-15+=+0) has the following solutons:

x%5B12%5D+=+%28b%2B-sqrt%28+b%5E2-4ac+%29%29%2F2%5Ca

For these solutions to exist, the discriminant b%5E2-4ac should not be a negative number.

First, we need to compute the discriminant b%5E2-4ac: b%5E2-4ac=%28-8%29%5E2-4%2A1%2A-15=124.

Discriminant d=124 is greater than zero. That means that there are two solutions: +x%5B12%5D+=+%28--8%2B-sqrt%28+124+%29%29%2F2%5Ca.

x%5B1%5D+=+%28-%28-8%29%2Bsqrt%28+124+%29%29%2F2%5C1+=+9.56776436283002
x%5B2%5D+=+%28-%28-8%29-sqrt%28+124+%29%29%2F2%5C1+=+-1.56776436283002

Quadratic expression 1x%5E2%2B-8x%2B-15 can be factored:
1x%5E2%2B-8x%2B-15+=+1%28x-9.56776436283002%29%2A%28x--1.56776436283002%29
Again, the answer is: 9.56776436283002, -1.56776436283002. Here's your graph:
graph%28+500%2C+500%2C+-10%2C+10%2C+-20%2C+20%2C+1%2Ax%5E2%2B-8%2Ax%2B-15+%29