just use your definitions:
a + d = 15
a + 4d = 21
subtract them
3d = 6
d = 2
mentally from a+d = 15, a = 13
sum(n) = (n/2)(2a + (n-1)d)
= 8(26 + 15(2))
= ...
The second term of a linear sequence is 15 and the 5th term is 21.Find the first term,the common difference and the sum of the first 16 term
1 answer