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Prove `1.4.7+2.5.8+3.6.9+.......` upto n terms `=(n)/(4)(n+1)(n+6)(n+7)` |
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Answer» Let `P(n):1.4.7+2.5.8+3.6.9+......+` upto n terms `(n)/(4)(n+1)(n+6)(n+7)` i.e., `P(n):1.4.7+2.5.8+3.6.9+......+n(n+3)(n+6)=(n)/(4)(n+1)(n+6)(n+7)` Step I For `n=1`, LHS of Eq. (i) `=1.4.7=28` RHS of Eq. (i) `=(1)/(4)(1+1)(1+6)(1+7)=(2.7.8)/(4)=28` LHS = RHS Therefore, P(1) is true . Step II Let us assume that the result is true for `n=k`. Then , `P(k):1.4.7+2.5.8+3.6.9+......+k(k+3)(k+6)=(k)/(4)(k+1)(k+6)(k+7)` Step III For `n=k+1`, we have to prove that `P(k+1):1.4.7+2.5.8+3.6.9+.....+k(k+3)(K+6)+(k+1)(k+4)(k+7)` `=((k+1))/(4)(k+2)(k+7)(k+8)` LHS =1.4gt7+2.5.8+3.6.9+......+k(k+3)(k+6)+(k+1)(k+4)(k+7)` =(k)/(4)(k+1)(k+6)(k+7)+(k+1)(k+4)(k+7)`[by assumption step ] `=(k+1)(k+7){(k)/(4)(k+6)+(k+4)}` `=(k+1)(k+7){(k^2+6k+4k+16)/(4)}` `=(k+1)(k+7){(k^2+10k+16)/(4)}` `=(k+1)(k+7){((k+2)(k+8))/(4)}` `=((k+1))/(4)(k+2)(k+7)(k+8)=RHS` This shows that the result is true for `n=k+1`. Hence, by the principle of mathematical induction , the result is true for all `n in N`. |
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