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Question

Question: Number of positive integral solution of x + y + z = 6 is...

Number of positive integral solution of x + y + z = 6 is

A

4 !

B

3 !

C

6!

D

10

Answer

10

Explanation

Solution

Number of positive integral solution of x+y+z = n is

Coefficient of xn in (x+x2 + x3 + ...)3 = Coefficient of xn (1+x+x2 + ...)3

= Coefficient of xn-3 in (11x)3\left( \frac{1}{1 - x} \right)^{3}

= Coefficient of xn-3 in (1-x)-3 = n1Cn3=n1C2n - 1C_{n - 3} =^{n - 1}C_{2}

Here n = 6

∴ Required solution = 5C2=105C_{2} = 10