Question

the first term of an ap is 25 and 9th term is -15 find the common difference and sum of the series​

Answer 1

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Here's your answer

Given that,

First term = a = 25

Ninth term = a₉ = -15

Common difference = ?

We know that,

aₙ = a+(n-1)d

Hence,

a₉ = a+(9-1)d

a₉ = a+8d

-15 = 25+8d

8d = -15-25

8d = -40

d = -5

Sₙ = n/2(a+aₙ)

S₉ = 9/2 [25+(-15)]

S₉ = 9/2 * 10

S₉ = 45

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