Oh maths...I'm not useless at maths, but I really don't have any need of complex equations in my job. As long as I know which equations are in which excel boxes I'm pretty much ok, which means I am not in need of any of today's apps, either for teaching or my own learning.
However, I think they all sound pretty cool and useful, so here they are in case you'd like to use them!
PhotoMath will use your phone's camera to give you the answer to any printed problem (well, they are still working on the really complex ones). It also shows you the working out, which is neat.
MyScript Calculator lets you write equations on the screen and will then solve them.
Desmos is a graphing tool which I in no way understand but I can make it create fairly pretty patterns. Desmos is available using a browser as well so you just need to go to the site to get started.
Enjoy your mathematics my friends!