Representations:
Let:
x - the first number
x + 1 - the second number
Step-by-step solving: Equations Transformable into Quadratic Equations
x(x + 1) = 90
x² + x = 90
x² + x - 90 = 0
(x - 9)(x + 10) = 0
x - 9 = 0
x = 9
x + 10 = 0
x = -10
The second root may be unacceptable at some point because of its sign, so we'll use the first root to substitute the original equation.
Check:
x(x + 1) = 90
9(9 + 1) = 90
9(10) = 90
90 = 90
True.
