$\sum_{\mathrm{r}=1}^{\mathrm{n}}{ }^{\mathrm{n}} \mathrm{C}_{\mathrm{r}} \cdot 3^{\mathrm{r}}=$

Question

Professor PC Thomas Classes · Answer

$\sum_{r=1}^n { }^nC_r 3^r = (1+3)^n - { }^nC_0 = 4^n - 1$.