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How many terms of the AP : 24, 21,18,... must be taken so that their sum is 78? |
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Answer» a=24 and d=21-24=-3 `S_n=n/2(2a+(n-1)d)` `78=n/2(2(24)+(n-1)(-3))` 156=n(48-3n+3) `n^2-51n+156=0` n(n-13)-4(n-13)=0 (n-4)(n-13) n=4,13 n can not be 13 so, n=3 |
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