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.