Question: In the expansion of $(1 + x + x^{2})e^{- x}$, the coefficient of $x^{2}$ is....

Question

In the expansion of $(1 + x + x^{2})e^{- x}$ , the coefficient of $x^{2}$ is.

Answer 1

$\frac{a - b}{r!}$

$\frac{a - br}{r!}$

Coefficient of

$( - 1)^{r}\frac{a - br}{r!}$ .

Question: In the expansion of \((1 + x + x^{2})e^{- x}\), the coefficient of \(x^{2}\) is....