# x^2 + 8x -78=-5 Solve by completing the square.

### 2 Answers | Add Yours

`x^2+8x-78=-5`

Add `78` to both sides.

`x^2+8x= 73`

In order to create a trinomial square on the left side of the equation, add a value to both sides that is equal to the square of half the coefficient of `x` . In this case, add `4^2` to both sides.

`x^2+8x+16=73+16`

`x^2+8x+16=89`

Now factor the perfect trinomial square.

`(x+4)^2 =89`

Take the square root of each side of the equation.

`(x+4)^(2/2) = +-sqrt89`

`x+4 =+-sqrt89`

`x= +sqrt89 -4`

`x=-sqrt89 -4`

**The complete solution is:**

**`x=sqrt89 - 4, -sqrt89-4` **

Solve each equation by completing the square.