Question

If 2x+5y = 15 and xy =8, find the value of 4x²+25y².
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Answer 1

2x+5y = 15

square both side,

(2x+5y)^2 = (15)^2

=> 4x^2 + 2×2x×5y + 25y^2 = 225

=> 4x²+25y² = 225 - 20 xy

=> 4x²+25y² = 225 - 160

=> 4x²+25y² = 65

so, the value of 4x²+25y² is 65