Question

4.If the sum of 1st 11 terms of an A.P is equal to sum of 1st 19 terms of that A.P then, Find the
sum of 1st 30 terms?
a) 5
b) 0
c) 13
d) -12
e) None of these

Answer 1

Answer:

e. none of these

Step-by-step explanation:

s₁₁=s₁₉

or,11/2[2a+10d]=19/2[2a+18d]

or,11/2×2[a+5d]=19/2×2[a+9d]

or,11a+55d=19+17d

or,19a-11a=55a-17d

or,8a=-116d

or,2a=29d

now,

s₃₀=30/2[2a+29d]

=15[29d+29d] (∵2a=29d)

=15[58d]

=870d

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