If 4x - 9y =10 and xy = -1, then find the value of 16x2+81y2.
Answer is 28
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(4x - 9y)^2 = 16x^2 + 81y^2 - 2.4x.9y
10×10 = 16x^2 + 81y^2 - 72xy
100 = 16x^2 + 81 y^2 - 72×1
16x^2 + 81y^2 = 100+72 = 172 Ans
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