Question

How do you solve `5x^2 - 90 = 0`?

Answer 1

`x = +3sqrt2" or " x = -3sqrt2`

Explanation:

Usually to solve a quadratic equation, we make it equal to 0.
However in this case, as there is no term in x, we can use a different method.

`5x^2 = 90 " "÷ 5`

`x^2 = 18 " find the" +- sqrt`

`x = +-sqrt18`

`x = +3sqrt2" or " x = -3sqrt2`

If the constant had been a perfect square we could have factorised as difference of two squares, but 18 is not a square.