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Question

Prove 2+5+8+11+...+(3n1)= 12 n(3n+1)nϵ N

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Solution

Let P(n) be true for n=m, that is, we suppose that
P(m)=2+5+8+11+...+(3m1)= 12 m (3m + 1)
Now P(m + 1) = P(m) + Tm+1
=12m(3m+1)+[3(m+1)1]
=12[3m2+m+6m+62]
=12[3m2+7m+4]
=12(m+1)(3m+4)
=12(m+1)[3(m)+1]
Above relation shows that P(n) is true for n = m + 1.
Also when n = 1,P(n) = 2 = 12(31+1)
n = 2, (P(n) = 2 + 5 = 7 = 122(32+1)
Above relation shows that P(n) is true for n = 1,2 etc.
Hence P(n) is universally true by mathematical induction.

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