Combinatorics

9000136901

Level: 
B
The sum \(\left({15\above 0.0pt 8} \right) +\left ({15\above 0.0pt 9} \right)\) equals to:
\(\left({16\above 0.0pt 9} \right)\)
\(\left({15\above 0.0pt 10}\right)\)
\(\left({15\above 0.0pt 7} \right)\)
\(\left({16\above 0.0pt 8} \right)\)
\(\left({30\above 0.0pt 17}\right)\)

9000136905

Level: 
B
For \(n\in \mathbb{N}\), \(n\geq 2\), the difference \(\left({n\above 0.0pt 2} \right) -\left ({ n\above 0.0pt n-2}\right)\) equals to:
\(0\)
\(\left (n + 2\right )\left (n + 1\right )\)
\(\left({n+2\above 0.0pt n} \right)\)
\(n^{2} - 1\)
\(\left({n\above 0.0pt n}\right)\)