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

Question

anonymous · Answer

a couple of ways to go about this.  Less interesting is the straight forward way of solving for x and y by adding the equations together to get:
$$2x = 40 \iff x = 20$$
Then y will be:
 $$20+y = 15 \iff y = -5$$
plugg and chug after that.

anonymous · Answer

$$(x-y)(x+y) = x^2-y^2$$

anonymous · Answer

@alchemista it doesnt work that y :o

anonymous · Answer

it does <.< his way is more interesting. you multiply, then add on an x^2.

angela210793 · Answer

@Alchemista is just x^2 multiplied with 2 not all tht

anonymous · Answer

i did that way but got stuck when i see the 2x^2

anonymous · Answer

$$2x^2-y^2 = x^2+x^2-y^2 = x^2+(x+y)(x-y)$$

anonymous · Answer

I misread it, yes indeed its not all multiplied by 2

anonymous · Answer

ohh i see what you guys try to say ^.^ thank you all xD