A: Because I didn't feel like it, that's why. And how dare you assume I'm bad at math. I'm not. Jesus Christ, you can't go anywhere on the Internet without running into the know-it-alls.
Besides, you never defined that expression as the function f(A) in the first place, so really, the simplest you could make that expression would have been A(PI[sup]5[/sup])/3.
Q: Why do Europeans insist on spelling math "maths" when mathematics is clearly a singular noun?