The sum of two numbers is 7 and the sum of their cubes is 133, find the sum of their squares.

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Solution

The correct option is C 29
Let a, b be the two numbers

.'. a + b = 7 and a³ + b³ = 133

(a + b)³ = a³ + b³ + 3ab (a + b)

=> (7)³ = 133 + 3ab (7)

=> 343 = 133 + 21ab => 21ab = 343 - 133 = 210

=> 21ab = 210 => ab= 10

Now a² + b² = (a + b)² - 2ab

= 7² - 2 x 10 = 49 - 20 = 29

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