Integration Question HP50g

[Fanboy]

I'm trying to integrate e^-(ax^2) dx from -inf to +inf. When I use PREVAL, it says unsigned inf error. When I use use equation editor to find the answer, it just keeps adding t in the expression. Wolfram alpha does it correctly:
https://www.wolframalpha.com/input/?i=%E...+to+%2Binf

I know I might have to make an assumption that the 'a' in my expression is greater than 0 but I don't know how to do so. So any suggestions?

Also, are all the integration functions equal or are some better than the other. For eg, for definite integrals, are PREVAL, INT, and using the equation editor methods all equal? or is one inherently better than the other?