SASMO 2017 Secondary 4 (Grade 10) Contest Question 19

“Double Factorial” n!! of a positive integer n is defined as
ee aE ae n=even
“tn x (n-2) XX 3X1, n=odd
For example,
SU=5x3x1,
6ll=6x4x2
Simplify
17! x 16! +16! x 15!!
1x2x3x..x 15x16"