Question
Find the differences between the largest values in the following: ${ }^{15} C_r-{ }^{11} C_r$

Answer

Greatest value of ${ }^{15} \mathrm{C}_{\mathrm{r}}$
Here $\mathrm{n}=15$, which is odd
Greatest value of ${ }^n C_r$ occurs at $r=\frac{n-1}{2}$ if $n$ is odd
$
\begin{array}{ll}
\therefore & \mathrm{r}=\frac{\mathrm{n}-1}{2} \\
\therefore & \mathrm{r}=\frac{15-1}{2}=7
\end{array}
$
$\therefore \quad$ Greatest value of ${ }^{15} \mathrm{C}_{\mathrm{r}}={ }^{15} \mathrm{C}_7=\frac{15 !}{7 ! 8 !}$
$
=\frac{15 \times 14 \times 13 \times 12 \times 11 \times 10 \times 9 \times 8 !}{7 \times 6 \times 5 \times 4 \times 3 \times 2 \times 1 \times 8 !}
$
$=6435$
Also, for greatest value of ${ }^{11} \mathrm{C}_{\mathrm{r}}$ $\mathrm{n}=11$, which is odd
$
\begin{aligned}
\therefore \quad \mathrm{r}=\frac{11-1}{2} & =5 \\
\therefore \quad{ }^{11} \mathrm{C}_5 & =\frac{11 !}{5 ! 6 !} \\
& =\frac{11 \times 10 \times 9 \times 8 \times 7 \times 6 !}{5 \times 4 \times 3 \times 2 \times 1 \times 6 !} \\
& =462
\end{aligned}
$
Difference between the greatest values of ${ }^{15} C_r$ and ${ }^{11} C_r=6435-462=5973$Find the differences between the largest values in the following: ${ }^{15} C_r-{ }^{11} C_r$

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