Question

Third term of AP is -17 & fifth term is13 find sum of 21 terms

Answer 1

Answer:

Step-by-step explanation: a3=-17

a+2d=-17....(1)

Similarly

a5=13

a+4d=13.....(2)

Subtract (1) from (2)

We get

2d=-4

d=-2

First term of an ap will be a+2d-2d=17-2(-2)=17+4=21=a

a n=a21=a +20d

= 21 +20(-2)=21-40=-19

Sn=n/2(a+an)

Therefore

S21=21/2(21-19)

21/2*2

=21 is your answer

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