Question

When 4 times a positive number is subtracted from the square of the number, the result is 5. Find the number.

Answer 1

5

Explanation:

x² - 4x = 5

x² - 4x - 5 = 0

the solution of a quadratic equation is

x = (-b ± sqrt(b² - 4ac))/(2a)

a = 1

b = -4

c = -5

x = (4 ± sqrt(16 + 20))/2 = (4 ± sqrt(36))/2

x1 = (4 + 6)/2 = 5

x2 = (4 - 6)/2 = -1

since we are looking only for a positive number, x=5 is the answer.