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Question

For every natural number n, n(n+1) is always

A
odd
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B
even
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C
divisible by 3
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D
divisible by 4
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Solution

The correct option is B even
The product of two consecutive numbers isalways even.We can prove it by principle of mathematicalinduction.Let P(n) be the statementP(n): n(n+1) is even. For n=1, P(n)=1(1+1)=2, which is even.i.e., P(1) is true.Assume P(n) is true for n=k,i.e., k(k+1)=k2+k is even. ...(1)We have to prove that P(k+1) is true wheneverP(k) is true.Consider (k+1)((k+1)+1) =(k+1)(k+2) =k2+2k+k+2 =k2+k+2(k+1) =2m+2(k+1) (Using (1)] =2(m+k+1)P(k+1) is a multiple of 2,i.e., P(k+1) is even.Thus, P(k+1) is true whenever P(k) is true.Hence, from the principle of mathematical induction,P(n) is true for every natural number.

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