CalculatorX is the best calculator available in market.It has some new and unique feature which distinguishes it from other maths help tools in market.The fact that it an even perform Integration and differentiation makes it a quiet handy in advance mathematical calculations.Some of its other features include-
-Simple user interface
-Very small in size
-Consumes very little RAM so it can easily run with other apps in background
-No Ads at all !!!
-Its free ,Most calculator with advance features are paid but CalculatorX is free.