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Question -

The sum of the squares of two consecutive multiple of 7 is 637. Find the multiples. [ICSE 2014]



Answer -

Let first multiple of 7 = 7x
Then second = 7x + 7
(7x)2 + (7x + 7) = 637
49x2 + 49x2 + 98x + 49 = 637
98x2 + 98x + 49 тАУ 637 = 0
98x2 + 98x тАУ 588 = 0
x2 + x тАУ 6 = 0 (dividing by 98)
x2 + 3x тАУ 2x тАУ 6 = 0
x (x + 3) тАУ 2 (x + 3) = 0
(x + 3) (x тАУ 2) = 0
Either x + 3 = 0, then x = -3, but not possible being negative
or x тАУ 2 = 0, then x = 2
Numbers will be 14, 21

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