One Calculator for Android is a scientific calculator app which includes features to graph functions, do operations on matrices, and convert between a whole load of different units.

One Calculator keeps a history of your calculations, so you can scroll back, see how you got various answers, and use them in the next step. Some functions (max, min, sum, avg, definite integral) work on series of numbers. The app helpfully guides you on how to input the correct notation, and this is simple enough to use. No shift buttons here, just long press for the alternative key functions.

There are a few things to watch out for. It gives answers to 10 decimal places, and also stores numbers like pi and e to that accuracy. So ln(e) evaluates to 0.9999999998 instead of 1. Be weary of blindly using this app to answer homework questions! Most calculators and calculator apps will recognise that ln(e) = 1, so this isn't such a good sign.

All the same One Calculator is a neat little tool for playing around with mathematics. It could also be handy in a pinch for doing matrix multiplications, and converting between units. Or just finding out how many hours there are in a year, or how your height or weight compares to somebody using a different system to you.

You can plot two functions of x on the plotter at the same time, so you can use it to show how cos(x) differs from 2cos(x), or cos(2x). Or try combing different functions, e.g. cos(tan(x)), to see what you get, and then try to think about why.