Apply sum/difference rule for indefinite integrals (#552)

Apply sum/difference rule for indefinite integrals (#552)

Use ∫[f(x)±g(x)] dx=∫f(x) dx±∫g(x) dx\int [f(x) \pm g(x)] \, dx = \int f(x) \, dx \pm \int g(x) \, dx.