Sum Arithmetic Progression in Haskell
My first comment would be to comment your code.

You need documentation so I know what your function is meant to do. "Sum any given finite Arithmetic Progression" would suffice if you passed in an ArithmeticProgression, but instead you pass in some bounds. Writing an arithmetic progression type would be simple and might even help a little, but you still need to document that.

I don't know what algorithm you're trying to use. You've evidently taken some maths for sums of arithmetic progressions and applied it to the problem, but there's no way for me to verify that the maths is correct or that the code implements it correctly without at least rudimentary explanation of what you did. I could reimplement it myself, but code should be verifiable much more easily than that.