The square of a number plus the number is 90. What is the number?

Question

Don't understand how to get the answer 

Answer 1

Hi Syvanna,

 

Let's call the number x

Then x^2 + x = 90 (we are given this in the question)

by subtracting 90 from both sides, we get

x^2 + x - 90 = 0

(x + 10)(x - 9) = 0

the solution set is  x = {-10, 9}

Actually, there are two numbers satisfying the condition stated in the question. If you plug them in, you will see.