Question

if x + y =10 and xy = 21, find 2(x^2 + y^2)
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Answer 1

$consider \: {x}^{2} + {y}^{2} \\ = {(x + y)}^{2} - 2xy \\ = {10}^{2} - 2 \times 21 \\ = 100 - 42 \\ = 58 \\ here \: {x}^{2} + {y}^{2} = 58 \\ 2( {x}^{2} + {y}^{2} ) = 2 \: \times 58 = 116$