Why doesn't DSolve not give good analytic solution?

So, I am new to Mathematica and everyone told me how useful it is. It has been quite a bit, but some of the answers I get always bug me. I am pretty sure I am not doing anything wrong. But when I used DSolve to solve a non-homogeneous differential equation, The solution has weird forms. Well, they could be equivalent, But should there be an option to see the solution in a better, less complex form?
I would know the answer, which will have like five terms but Mathematica would give more than 10 terms. I tried some online calculators and they are better at solving everything symbolically. My teacher still prefers Mathematica for homework. I have experienced that Mathematica also gives weird answers to integrations. And yes, I have tried simplified the expressions. I am attaching a file. If the answers can be simplified in terms of sines and cosines, by method of undetermined coefficients, It would be a great help.

Two examples of needlessly complex answers to an ODE.nb (1.61 MB)

Compare this

Simplify[DSolve[{1/2 y''[x]+20 y'[x]+10^3 y[x]==100 Sin[60 x]+200 Cos[40 x]},y[x],x]]

with your first example

Thanks, Only if I knew that you should always write in terms of fractions, not decimals. (Face Palms)