Find a if 17th and 18th terms in the expansion of (2+a)^50 are equal.

The 17th term is 50C16*2^34*a^16
The 18th term is 50C17*2^33*a^17

The ratio of these two terms is (17th term)/(18th term) = 1/a. We want the value of this to be 1, so
.. A = 1


My calculator says
.. 50C16 = 4,923,689,695,575
.. 50C17 = 9,847,379,391,150
the latter being exactly 2 times the former.