Question

the sum of 25+28+31+......100

Answer 1

Hy there!
a = 25 here a d d = 3

So we have:

An = a + (n - 1) d

100 = 25 + (n-1) 3
78/3 = n

so, n = 26.

Now, Sn = 26/3 ( 25 + 100)
Sn = 13 ( 125)

Hence, Sn = 1625

hope this helps.
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Answer 2

Given,

➡a=25

➡an=100

➡d=a2-a1➡28-25=3

➡an=a+(n-1)d

➡100=25+(n-1)3

➡75/3=n-1

➡25=n-1

➡n=26

➡Sn=n/2(a+an)

➡Sn=26/2(25+100)

➡Sn=13×125

➡Sn=1625