Solution to Prove that for all integer n3, P(n+1,3)−P(n,3)=3P(n,2). - Sikademy
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Prove that for all integer n3, P(n+1,3)−P(n,3)=3P(n,2).

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Here's the Solution to this Question

We shall prove by mathematical induction.

For n=3,

P(4,3)-P(3,3)=24-6=18=3(6)=3P(3,2)

Thus it is true for n=3

Assume it is true for n=k,k\geq3. Then

P(k+1,3)-P(k,3)=3P(k,2)

We shall show that it is true for n=k+1,k\geq 3

That is, P(k+2,3)-P(k+1,3)=3P(k+1,2)

P(k+2,3)-P(k+1,3)=\frac{(k+2)!}{(k-1)!}-\frac{(k+1)!}{(k-2)!}=k(k+1)(k+2)-k(k-1)(k+1)\\ =3k(k+1)

For the RHS

3P(k+1,2)=3\left(\frac{(k+1)!}{(k-1)!}\right)=3k(k+1)

Since LHS=RHS, then it is true for n=k+1,k\geq 3

Hence, P(n+1,3)-P(n,3)=3P(n,2), n\geq 3


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