Question

If x+y=15 and x-y=25, then what is the value of 2x^2 - y^2?

A) 425
B) 775
C) 825
D) 1,575
E) 1,625 ...?

Answer 1

The answer is B as since x = 20 and y = -5

Answer 2

x+y = 15
x-y = 25
------------
2x = 40
x = 40/2
x = 20

y = 15-x
y = 15-20
y = -5

2x² - y² =
2*20² - (-5)² =
2*400 - 25 =
800 - 25 =
775