Question

What are two numbers that multiply to -20 and add up to 1?

Answer 1

xy = -20
x + y = 1

x + y = 1
x - x + y = -x + 1
y = -x + 1

xy = -20
x(-x + 1) = -20
x(-x) + x(1) = -20
-x² + x = 20
-x² + x + 20 = 0
-1(x²) - 1(-x) - 1(-20) = 0
-1(x² - x - 20) = 0
-1 -1
x² - x - 20 = 0
x = -(-1) ± √((-1)² - 4(1)(-20))
2(1)
x = 1 ± √(1 + 80)
2
x = 1 ± √(81)
2
x = 1 ± 9
2
x = 0.5 ± 4.5
x = 0.5 + 4.5 or x = 0.5 - 4.5
x = 5 or x = -4

x + y = 1
5 + y = 1
- 5 - 5
y = -4
(x, y) = (5, -4)
or
x + y = 1
-4 + y = 1
+ 4 + 4
y = 5
(x, y) = (-4, 5)

The two numbers that add up to 1 and multiply to -20 are -4 and 5.