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by sacx » Mon Apr 27, 2009 4:25 am
x+y = a...............(1)

x-y = b.................(2)

add eqn 1 & 2

2x = a+b
x = (a+b)/2

Sustitute the value of x in eqn 1

(a+b)/2 + y = a

y = a- (a+b)/2
y = a - a/2 - b/2
y = a/2-b/2
y = (a-b)/2

2xy = 2*(a+b)/2*(a-b)/2
= (a^2-b^2)/2

Choose A
SACX
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by GMAT12 » Mon Apr 27, 2009 5:40 am
(x+y)^2-(x-y)^2=a^2-b^2
x^2+2xy+y^2-x^2+2xy-Y^2=a^2-b^2
4xy=a^2-b^2
2xy=(a^2-b^2)/2
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