Problema Solution
find the sum of the following series whose difference is in AP:- 5+7+11+17+25+......upto 30 terms.
Answer provided by our tutors
a0 = 5
a1 = a0 + 1*2
a2 = a1 + 2*2
a3 = a2 + 3*2
a4 = a3 + 4*2
a5 = a4 + 5*2
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an-1 = an-2 + (n - 1)*2
an = an-1 + n*2
Lets sum up he left sides and the right sides of the equations:
a0 + a1 + ... + an = 5 + a0 + ... + an-1 + 2(1 + 2 + ...+ n)
S = a0 + a1 + ... + an
S = 5 + S - an + 2*(n/2)(1 + n)
an = 5 + n(n + 1)
The sum of the first 30 terms is:
5 + 1*2 + 5 + 2*3 + 5 + 3*4 + .... + 5 + 30*31 =
= 5*30 + 1*2 + 2*3 + ...+29*30 + 30*31 = 150 + 9920 = 10,070.