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Mod - Set 11 Q7

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Mod - Set 11 Q7

by arocks » Sun Oct 07, 2007 11:22 pm
If x<0, then (-x*&#9474;x&#9474;)^1/2 is

A -x
B -1
C 1
D x
E x^1/2

IMO D. But the OA is A. Pls explain. Thanks.
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by jay2007 » Mon Oct 08, 2007 12:45 pm
Let a = -x say.
(-x*|x|) = (a*a)^1/2
= a
= -x.

Therefore answer = A.

Pls. correct me if i am wrong.
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by ldoolitt » Mon Oct 08, 2007 1:33 pm
jay2007 wrote:Let a = -x say.
(-x*|x|) = (a*a)^1/2
= a
= -x.

Therefore answer = A.

Pls. correct me if i am wrong.
Right. Or you could say

since x<0, we know that |x| = -x [ie |-3| = -(-3) = 3]

thus we have

(-x * -x )^1/2
((-x)^2)^1/2
(-x)^1/2 * 2
-x^1
-x

by rule of exponents
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by lalitgmat » Mon Oct 08, 2007 2:00 pm
The rule is:
sqaure root of ( square of x ) can be +x or -x as (+x) *(+x) = (-x)(-x).
In this case x<0, hence we take -x.

Had the question being all same, except if x > 0, ans would have been (D)
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