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Question

Question 21 (i)
Find the sum:
1 + (-2) + (-5) + (-8) + . . . + (-236)

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Solution

Given,
a = 1, d = - 3 and l = -236
an=a+(n1)d236=1+(3)×(n1)3×(n1)=237n=1+2373=1+79=80.
The number of terms in the AP is 80.
Sum of n terms of an AP,
Sn=n2[a+l] [where a is the first term and l is the last term]
=802(1+(236)) [n=80]
=40×(235)=9400

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