Problem following working in binomial theorem proof by induction

Question

I am working through a (rather old) pure maths book as a hobby and have got to a section showing proof of the binomial theorem using induction. Part way through the working the book says:

$_kC_r + _kC_{r-1} = \frac{k!}{r!(k-r)!} + \frac{k!}{(r-1)!(k-\{r-1\})!}$

which I can follow. But I cannot see how the following line is derived:

$= \frac{k!}{r!(k-r+1)!}\{(k-r+1)+r\}$

Answer 1

$\frac{k!}{r!(k-r)!} + \frac{k!}{(r-1)!(k-\{r-1\})!}=\frac{k!(k-r+1)}{r!(k-r)!(k-r+1)} + \frac{k!}{(r-1)!(k-r+1)!}=\frac{k!(k-r+1)}{r!(k-r+1)!} + \frac{k!\cdot r}{r\cdot (r-1)!(k-r+1)!}=\frac{k!(k-r+1)}{r!(k-r+1)!} + \frac{k!\cdot r}{r!(k-r+1)!}=\frac{k!}{r!(k-r+1)!}\{(k-r+1)+r\}$

Answer 2

In general $$\frac{a}{bc}+\frac{a}{bd} = \frac{a(d+c)}{bcd}$$ and here

• $a=k!$,
• $b=(r-1)!(k-r)!$,
• $c=r$,
• $d=k-r+1$.