Question

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For how many term 6+9+12+... the sum is 81.
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Answer 1

Answer:

6+9+12+15+18+21=81

so there are total 6 terms to get the sum 81

hope it will help u..

remember to make it as brajnlist answer...

Answer 2

Step-by-step explanation:

first term = 6

common difference = 9 - 6 = 3

sum = 81

then,

$sn = \frac{n}{2} (2a + (n - 1)d) \\ \\ 81 = \frac{n}{2} (2 \times 6 + (n - 1)3) \\ \\ 81 \times 2 = n(12 + 3n - 3) \\ \\ 162 = 9n + 3 {n}^{2} \\ \\ 3 {n}^{2} + 9n - 162 = 0 \\ \\ 3( {n}^{2} + 3n - 54) = 0 \\ \\ {n}^{2} + 3n - 54 = 0 \\ \\ { n}^{2} + 9n - 6n - 54 = 0 \\ \\ ( {n}^{2} + 9n) - (6n + 54) = 0 \\ \\ n(n + 9) - 6(n + 9) = 0 \\ \\ (n - 6)(n + 9) = 0 \\ \\ n - 6 = 0 \: or \: n + 9 = 0 \\ \\ n = 6 \: or \: n \: = - 9$