How many terms of the A.P. 3, 6, 9, 12, 15..must be taken to make the sum 108?

6

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7

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8

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36

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Solution

The correct option is D $8$
$\frac{n}{2} [2.3 + (n - 1) 3] = 108$
$6 n + 3 n^{2} - 3 n = 216$
$3 n^{2} + 3 n - 216 = 0$
$x^{2} + n - 72 = 0$
$(x + 9) (x - 8) = 0$
$x = 8$

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