Equation: Let the integer be x.
x² + x = 90
Equate to zero to solve the quadratic equation:
x² + x - 90 = 0
Factor:
(x + 10) (x - 9) = 0
Solve for roots (x):
x + 10 = -10 x - 9 = 0
x = -10 x = 9
Choose the positive root, x = 9, the integer.
To check:
(9)² + 9 = 90
81 + 9 = 90
90 = 90
