SOLUTION: how do so solve x^2-81=0 using the quadratic formula?

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Question 374499: how do so solve x^2-81=0 using the quadratic formula?
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
x%5E2-81=0 Start with the given equation.


Notice that the quadratic x%5E2-81 is in the form of Ax%5E2%2BBx%2BC where A=1, B=0, and C=-81


Note: x%5E2-81=0 is really x%5E2%2B0x-81=0


Let's use the quadratic formula to solve for "x":


x+=+%28-B+%2B-+sqrt%28+B%5E2-4AC+%29%29%2F%282A%29 Start with the quadratic formula


x+=+%280+%2B-+sqrt%28+%280%29%5E2-4%281%29%28-81%29+%29%29%2F%282%281%29%29 Plug in A=1, B=0, and C=-81


x+=+%280+%2B-+sqrt%28+0-4%281%29%28-81%29+%29%29%2F%282%281%29%29 Square 0 to get 0.


x+=+%280+%2B-+sqrt%28+0--324+%29%29%2F%282%281%29%29 Multiply 4%281%29%28-81%29 to get -324


x+=+%280+%2B-+sqrt%28+0%2B324+%29%29%2F%282%281%29%29 Rewrite sqrt%280--324%29 as sqrt%280%2B324%29


x+=+%280+%2B-+sqrt%28+324+%29%29%2F%282%281%29%29 Add 0 to 324 to get 324


x+=+%280+%2B-+sqrt%28+324+%29%29%2F%282%29 Multiply 2 and 1 to get 2.


x+=+%280+%2B-+18%29%2F%282%29 Take the square root of 324 to get 18.


x+=+%280+%2B+18%29%2F%282%29 or x+=+%28-0+-+18%29%2F%282%29 Break up the expression.


x+=+%2818%29%2F%282%29 or x+=++%28-18%29%2F%282%29 Combine like terms.


x+=+9 or x+=+-9 Simplify.


So the solutions are x+=+9 or x+=+-9


If you need more help, email me at jim_thompson5910@hotmail.com

Also, feel free to check out my tutoring website

Jim