Question

find the sum of the following ap 10+15+20...upto100​

Answer 1

10+15+20.......100

take 5 common

= 5(2+3+4+5.....20)

= 5 (20x21/2 - 1)

= 5 (210 -1)

= 5 x 209

= 1045

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Answer 2

10+15+20 upto 100 terms

this series is in Ap

a= 10

d= 5

n= 100

then

n th term = a+(n-1) d

= 4950 = l

sum of 100 terms.= (n/2)(a+l)

= 50× 4960

= 248000

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