${ }^{50} \mathrm{C}_{20}+30 \cdot{ }^{50} \mathrm{C}_{29}$

${ }^{50} \mathrm{C}_{19}+30 \cdot{ }^{49} \mathrm{C}_{19}$

${ }^{50} \mathrm{C}_{20}+30 \cdot{ }^{49} \mathrm{C}_{20}$

${ }^{50} \mathrm{C}_{20}+30 \cdot{ }^{49} \mathrm{C}_{19}$

$(-1)^n \frac{1 \cdot 3 \cdot 5 \ldots(2 n-1)}{2^n n!} 3^n$

$(-1)^{n+1} \frac{1 \cdot 3 \cdot 5 \ldots(2 n+1)}{2^{n+2}(n+2)!} 3^{n+2}$

$(-1)^{n+1} \frac{1 \cdot 3 \cdot 5 \ldots(2 n-1)}{2^n n!} 3^{n+1}$

$(-1)^{n+1} \frac{1 \cdot 3 \cdot 5 \ldots(2 n+1)}{2^{n+3}(n+2)!} 3^{n+1}$

$a b=-1$

$a b=1$

$a^5 b^5=-2$

$a^5 b^5=2$

47

48

24

23