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If the coefficient of x^r (0<=r<=n-1) in...

If the coefficient of `x^r (0<=r<=n-1)` in expression `(x+2)^(n-1)+ (x+2)^(n-2) (x+ 1) +(x+1)^(n-1)` is `b(n,r)` then `(b(10,7))/10` is equal to

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n-1=a
`S=(x+2)^a+(x+2)^(a-1)(x+1)+(x+2)^(a-2)(x+1)^2+..(x+1)^9`
`a=9,r=7`
`S=(x+2)^9+(x+2)^8(x+1)+(x+2)^7(x+1)^2..(x+1)^9`
`S=(x+2)^9[((x+1)/(x+1))^10-1)/((x+1)/(x+2)-1)`
`S=((x+1)^10-(x+2)^10)/(-1)`
Coefficient of `x^7=(10C_3-2^3 10C_3)/(-1)`
`10C_3(7)`
...
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