Problem of the Day: 7/8/2011

Which of these is the result of (2x+5)2 - (2x-5)2?
1. 40x2
2. 40x
3. 50x
4. 40
The correct answer is 2.

First, I simplified the problem,(2x+5)2 - (2x-5)2 = (4x2 + 25 + 20x) - (4x2 + 25 - 20x). Second, I solved for the answer, (4x2 + 25 + 20x) - (4x2 + 25 - 20x) = 40x.

Explanation: (2x+5)2 - (2x-5)2 = (4x2 + 25 + 20x) - (4x2 + 25 - 20x) = 40x.