If x² + y² = 58 and x + y = 10, then what is the value of xy?
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Correct answer: A
Explanation
Given:
x² + y² = 58
x + y = 10
By the formula:
(x + y)² = x² + y² + 2xy
(10)² = 58 + 2xy
100 = 58 + 2xy
2xy = 100 − 58
2xy = 42
xy = 21