It took me a while to understand the name of this app. But then I realised it's calledCalculator cubed because it's three calculators in one app. So what, I'm mathematically (and logically) challenged, OK?
For this reason, I would not do justice to Calculator³. Suffice to say that, as it comprises a scientific, a graphing and a programmer calculator, it's one of the most comprehensive out there. Best of all, it drops from 1.79₺ to FREE!