Aspire Faculty ID #15285 · Topic: JAMIA MCA 2016 · Just now
JAMIA MCA 2016

The coefficient of $x^4$ in expansion of $(1 + x + x^2 + x^3)^{11}$ is …

Solution

We can write generating function $f(x) = (1 + x + x^2 + x^3)^{11}$. Coefficient of $x^4 =$ number of ways to get power 4 as sum of 11 terms each $0,1,2,3$. By multinomial expansion, coefficient of $x^4 = {}^{11}C_4 + 10{}^{11}C_3 + 6{}^{11}C_2 + {}^{11}C_1 = 990$.

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