Question

How many terms of the AP -9,-3must be taken so that their sum is 66?

Answer 1

AP: -9,-3,3,9.....

Here,

a = -9

d = 6

Sn = n\2(2a+(n-1)d)

Sn=66

∴ 2*66=n(2*-9+(n-1)6)

132 = n(-18+6n-6)

132 = -24n+6n²

6n²-24n-132 = 0

n² - 4n - 22 = 0

This cannot be further solved therefore it is not possible for tis AP to get a sum of 66.

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