21-260: Edits to Online Assignment #1

I've made a couple corrections to the problems on Wednesday's online assignment. I know some of you (75 according to Blackboard) have already attempted these problems, but I don't think should create too many problems. Here are the changes I've made:

1. In problem #4, I've changed "dy/dx" to "dy/dt", since the variable on the right hand side of the equations is t, not x.
2. In problem #5, I've changed "f(t)" to "f(x)", since the independent variable in the differential equations is x.