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Question
Find the sum of the following APs.
−37, −33, −29, …, to 12 terms.
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Solution
−37, −33, −29, …, to 12 terms
For this A.P.,
a = −37
d = a2 − a1
= (−33) − (−37)
= −33 + 37
= 4
n = 12
We know that,
Sn = `n/2 [2a+(n - 1) d]`
S12 = `12/2 [2(-37)+(12 - 1) × 4]`
= 6[-74 + 11 × 4]
= 6[-74 + 44]
= 6(-30)
= -180
Thus, the sum of first 12 terms is -180.
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