Question

How do you find the first term of the arithmetic sequence given Sn = 781, d = 3, n = 22?

Answer 1

here
Sn=781,d=3,n=22,a=?

Sn=n/2[2a+(n-1)×d]
781=22/2[2a+(22-1)×3]
781=11(2a+21×3)
781/11=2a+63
71-63=2a
2a=8
a=8/2
a=4