Hi,
Probably it does exceed the maximum number of characters to be entered.
Note Wolfram Alpha is powered by Wolfram Mathematica. However, Alpha and Mathematica are slightly different for math. Wolfram Alpha accepts variables a_1 while Wolfram Mathematica does not.
If you upgraded to pro, you also have access to a chat, where they put you in contact with someone from Wolfram Alpha to help you solve this problem directly.
Hope this helps,
Nico.
