Advanced Math practice question

Answer: 13

Expand the square: (x + y)² = x² + 2xy + y². So x² + y² = (x + y)² − 2xy = 25 − 2(6) = 25 − 12 = 13. (For instance x = 2, y = 3 satisfies both conditions and gives 4 + 9 = 13.)

Common wrong answers

If you answered 19

19 is 25 − 6, subtracting xy once. The expansion of (x + y)² contains 2xy, so subtract 12.

If you answered 25

25 is (x + y)², which includes the extra 2xy. Subtract 2xy = 12 to get x² + y².

If you answered 37

37 is 25 + 12, adding 2xy instead of subtracting it. (x + y)² is BIGGER than x² + y² by 2xy.

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