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Question:

If x - y = 25 and xy = 78, then what is the value of x² + y²?

Explanation:

Given, x - y = 25

Squaring both sides, we get

x² + y² - 2xy = 625

⇒ x² + y² - 156 = 625 [since xy = 78]

⇒ x² + y² = 625 + 156 = 781

Hence, option (b).