Math, asked by bhideaniruddha551, 8 months ago

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

Answers

Answered by jagatpaljagat3844
0

Step-by-step explanation:

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Answered by mysticd
1

 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}

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