Question

In an A. P. if the 5th and 12th terms are 30 and 65 respectively, what is the sum of first 20 terms?

Answer 1

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Answer 2

a+4d=30
a+11d=65
by substitution method
a=10
d=5
a20=10+19×5
=10+95=105
s20=10 (10+105)
=10 (115)=1150
s20=1150