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Junior | Next Rank: 30 Posts
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alg

by d0nw0ng » Mon Oct 08, 2007 11:52 am
if x isnt 0, then (x+1/x)^2 - (x-1/x)^2 =

a) 1/x
b) x
c) 1
d) 2
e) 4
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by arocks » Mon Oct 08, 2007 12:08 pm
(x+1/x)^2 - (x-1/x)^2 = ?

Let (x+1/x)=a
and (x-1/x)=b

so, the eqn becomes a^2-b^2
we know a^2-b^2 = (a+b) (a-b)
so, [(x+1/x)+(x-1/x)] [(x+1/x)-(x-1/x)]
or 2x*2/x = 4

Ans: E
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by ldoolitt » Mon Oct 08, 2007 1:57 pm
Or you could just expand the two.

(x+1/x)^2 - (x-1/x)^2
= (x^2 + 2 + 1/x^2) - (x^2 - 2 + 1/x^2)
= 4
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