Question

if sum of the first 15 term of an ap is 150 and its first term is 17 find 20th term

Answer 1

S15 = 150
and a = 17
15/2(2(17) + (15-1)d) = 150
(2(17) + (15-1)d) = 150*2  /15
34 + 14d = 20
14d = 20 - 34 
14d = -14 
d=-1

a20 will be
a + (20-1)d 
17 + 19(-1)
17 - 19
= -2

Answer 2

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