Question

If $9^{th}$ and ${19}^{th}$ terms of an AP are 35 and 75 respectively, then ${20}^{th}$ term is -

• A. 80
• B. 78
• C. 81
• D. 79

Answer 1

The correct option is D

79

If a be the first term and d be the common difference of the AOP , then ,

$T_9=a+8d=35$

$T_{19}=a+18d=75$

Subtracting these equations , we get

$-10d=-40$

$d=4$

Therefore, $a=3$

$T_{20}=3+19\times 4=79$