Question:
If $7^{\text {th }}$ and $13^{\text {th }}$ terms of an AP be 34 and 64 respectively then find its $18^{\text {th }}$ term.
Solution:
Given: $7^{\text {th }}$ term is 34 and $8^{\text {th }}$ term is 64
To find: find its $18^{\text {th }}$ term
34 = a + 6d ………….equation1
64 = a + 12d ………… equation2
Subtract equation1 from equation2 we get
d = 5
Put in equation1 we get
a = 4
So $18^{\text {th }}$ term is
4 + 17 × 5 = 89
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