Question

an A P has 21 terms the sum of 10 ,11th and 12th term is129 and the sum of the last three us 237 . find the ap

Answer 1

according to ques
A10+A11+A12 = 129
a+9d+a+10d+a+11d = 129
3a+30d = 129
3(a+10d) =129
a+10d = 129/3
a+10d = 43.....(1)

A19+A20+A21=237
a+18d+a+19d+a+20d = 237
3a+ 57d=237
3(a+19d)=237
a+19d=237/3
a+19d=79........(2)

(2)-(1)
a+19d=79
-(a+10d)=(-43)
9d=36
d=36/9
d=4

putting value of d in equation (1)
a+10d=43
a+10(4)=43
a+40=43
a=43-40
a=3

AP SERIES IS 3,7,11,15,....