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kaplan roots

by resilient » Wed Apr 02, 2008 12:53 pm
rootx 4x9+9x16/25


a. 18/5
b.12root5/5
c. 3root26/5
d.2root3/5
e.6root5/5

qa is e i am getting a..

can someone show step by step
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by Stuart@KaplanGMAT » Wed Apr 02, 2008 1:16 pm
Just to clarify the question, the whole thing is under the root sign.

In other words, it's:

root ((4*9 + 9*16)/25)

We know that root (a/b) = roota/rootb. So, our first step in simplifying is to rewrite the expression as:

root(4*9 + 9*16)/root25 = root(4*9 + 9*16)/5 = root(36 + 144)/5

Choice (a) is a common trap designed for people who don't fully understand explonent/root rules. Here are the basics:

roota * rootb = root(ab)
roota/rootb = root(a/b)

HOWEVER:

roota + rootb does NOT= root(a+b)
and
roota - rootb does NOT= root(a-b)

So, we CANNOT say that root(36 + 144) = root36 + root144 = 6 + 12 = 18.

The proper way to solve is to add them together THEN take the root.

So:

root(36 + 144)/5 = root180/5 = root(36*5)/5 = root36*root5/5 = 6root5/5: choose (e).
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