First slide
Binomial theorem for positive integral Index
Question

The coefficient of x4in the expansion of (1+x+x2)10 is__________

Moderate
Solution

 General term =10!α!β!γ!×xβ+2γ, where α+β+γ=10

 For coefficient of x4,β+2γ=4

So, the possible sets will be

γ=0,β=4,α=6  10!6!4!0!=210γ=1,β=2,α=710!7!2!1!=360 γ=2,β=0,α=810!8!0!2!=45    

Total = 615

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