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Find the value of x² + y² if x + y = 10 and xy = 21:

A48
B58
C79
D100
Answer & Solution
Correct answer: B. 58
Use the identity (x + y)² = x² + 2xy + y². So x² + y² = (x + y)² − 2xy = 10² − 2·21 = 100 − 42 = 58.
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