Question

what is the sum of the first 20 terms of the arithmetic series if a1=5 and a20=62?

Answer 1

tn=a+(n-1)d

a = 5

a20 = 62 means a +19d=62

19d=62-5

19d=57

d=3

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

=20/2 (10+19(3))

=10(10+57)

=670