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. We have **So x² + y² = (x + y)² − 2xy. That is **So x² + y² = 10² − 2·21. That is **So x² + y² = 100 − 42. So **So x² + y² = 58.**.

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