GRE Math Problem 14

Compare two values:

Column A	Column B
	x+y-7

Solution for GRE Math Problem 13:

Let's use formula for the squares of sum and difference:

(a+b)²=2²+2ab+b²

(a-b)²=a²-2ab+b²

It seems that column A is bigger by value of 4ab. But wait! What if ab<0? Then Column B is bigger. So, it is impossible to determine, which column is greater. Answer:D

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