If x + y = 9 and x – y = 3, then x^2 - y^2 = Choose the most appropriate option:  A) -4  B) 4  C) 10  D) 27  E) 40
The fastest approach has already been shown. Here's one more option.   Given:   x + y = 9   x – y = 3   Add the two equations to get: 2x = 12, which means x = 6   If x = 6, we can plug that value into either equation, to conclude that y = 3   If x = 6 and y = 3, then x² - y² = 6² - 3² = 27   Answer: 27
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If x + y = 9 and x – y = 3, then x^2 - y^2 =