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The value of k for which the lines 5x+7y=3 and 15x + 21y = k coincide is (a) 9 (b) 5 (c) 7 (d) 18

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The value of k for which the lines 5x+7y=3 and 15x + 21y = k coincide is (a) 9 (b) 5 (c) 7 (d) 18

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5x + 7y =3 ........1

15x + 21y = k........2

multiply equation 1 with 3

3 * ( 5x + 7y =3)

15x +21y =9.....3

now subtract equation 2 from 3

15x + 21y =9

- 15x +21y = k

on subtracting we get k =9 . 

Since they are coincident lines. k = 9

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