4- If 3x + 4y = 12 and x y = 11, then find 9x2 + 16y2
5- If a2 + b2 = 10 and ab = 5, find the value of (a+ b)2
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(3x +4y)^2 = 9x^2 + 16 y^2 + 2 .3x.4y = 12^2
9x^2 + 16y^2 = 12^2 - 24xy
= 144- 24×11 = 144 - 264 = - 120
(a+b)^2= a^2 + b^2 + 2ab = 10 + 2×5 = 10 + 10 = 20
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