Question

in an AP if an = 9-5n, find the sum of first 20 terms

Answer 1

Step-by-step explanation:

hope you got your answer.

Answer 2

$Given \:in \: an \: A.P : a_{n} = 9 - 5n$

$i) If \: n = 1$

$then \: First \:term (a) = 9 - 5\times 1$

$= 9 - 5$

$= 4$

$Now, a = 4, a_{n} = 9 - 5n$

$and \: n = 20$

$\boxed{\pink{Sum \:of \:n \:terms (S_{n})= \frac{n}{2}(a + a_{n}) }}$

$Sum \:of \: first\: 20 \:terms = S_{20}$

$= \frac{20}{2}( 4 + 9 - 5n )$

$= 10( 13 - 5 \times 20)$

$= 10( 13 - 100)$

$= 10 \times (-87)$

$= -870$

Therefore.,

$\red{Sum \:of \: first\: 20 \:terms} \green {= -870}$

•••♪