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Question -

If 3x -7y = 10 and xy = -1, find the value of 9x2 + 49y2



Answer -

3x – 7y = 10, xy = -1
3x -7y= 10
Squaring both sides,
(3x – 7y)2 = (10)2
⇒ (3x)2 +(7y)2 – 2 x 3x x 7y = 100
⇒ 9X2 + 49y2 – 42xy = 100
⇒ 9x2 + 49y2 – 42(-l) = 100
⇒ 9x2 +49y2 + 42 = 100
∴ 9x2 +49y2 = 100 – 42 = 58

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