Math, asked by Tara5678, 1 year ago

find sum of last 15 terms of ap with a=2 and d=2 the total terms are 25

Answers

Answered by gaurav2013c

a = 2

d = 2

n = 25

L = a + (n-1) d

L = 2 + (25 - 1)2

L = 2 + 24×2

L = 2 + 48

L = 50

Now,

Arrange the terms in descending order.

50, 48, 46......

a = 50

d = - 2

S25 = 25/2 [ 2 × 50 + (25-1) (-2)]

= 25 /2 [ 100 + 24 (-2)]

= 25/2 [ 100 - 48]

= 25 × 52 / 2

= 25 × 26

= 400

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