If {9^{th}} term of an A.P. is 35 and {19^{th}} is 75 , then its {20^{th}} terms will be

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8. Sequences and Series

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If the ${9^{th}}$ term of an $A.P.$ is $35$ and ${19^{th}}$ is $75$, then its ${20^{th}}$ terms will be

A

$78$

B

$79$

C

$80$

D

$81$

Solution

(b) ${T_9} = a + 8d = 35$ and ${T_{19}} = a + 18d = 75$

Solving the equations, we get $d = 4$ and $a = 3$

Hence ${20^{th}}$ term of $A.P.$ ,

$a + 19d = 3 + 19 \times 4 = 79$.

Standard 11

Mathematics

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