# `(2r - 5)^2 = 81` Solve the equation.

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### 2 Answers

To solve quadratic equations like these, you have to square root both the left hand side and the right hand side of the "=" sign and like so:

`sqrt((2r-5))^2 = sqrt(81)`

`sqrt(81)= 9 and -9 `

Then equate the equations and solve:

`2r-5=9 ` and `2r-5=-9 `

Solve the first part like so:

`2r-5+5=9+5` (adding 5 both sides to get rid of 5 from the left hand side)

`2r=14 `

Divide both sides by 2:

` r=7 `

Solve the second part like so:

`2r-5+5=-9+5 ` (adding 5 both sides to get rid of 5 from the left hand side)

` 2r=-4 `

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` `

Divide both sides by 2:

` r=-2`

I would advise you to plug the two r values back in the main equation and see if both sides agree.

Let me know if you need help with this

### User Comments

square root both sides:

2r - 5 = +- 9

add 5 to both sides:

2r = 14 , 2r = -4

Divide both sides of both equations by 2:

r = 7 , r = -2