If first term of an A.P. be 10 , last term is 50 and sum of all terms is 300 , then number of terms

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8. Sequences and Series

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If the first term of an $A.P. $ be $10$, last term is $50$ and the sum of all the terms is $300$, then the number of terms are

A

$5$

B

$8$

C

$10$

D

$15$

Solution

(c) Given that first term $a = 10$,

last term $l = 50$ and sum $S = 300$

$\therefore \;\;S = \frac{n}{2}(a + l)$

$ \Rightarrow $ $300 = \frac{n}{2}(10 + 50)$

$ \Rightarrow $$n = 10$.

Standard 11

Mathematics

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