As an illustration of the problem of Obsessive-compulsive Disorder, i is pretty good. As a math resource, i is less than worthless.
For example, the Constants menu has seven submenus, titled Constants this app-IV and Useful Constants this app-III. As you can see, the titles give no hint of where you'll find the constant you're looking for. What's there is a mishmash of physics and chemistry constants with many important ones missing. They are arranged neither alphabetically nor by subject matter. And Constants II and III are the same!
Similar problems abound throughout. There are tons of formulas and diagrams you will never need, while many important basic ones are missing. At least once the end of a formula was clipped off the page.
It's written in a mix of German and English using (apparently) German math notation. For example ":" is used for division rather than "/".
Even for $0, i is a rip-off, because of the price you'll pay in headaches and wasted time.
i is so confusing it will make you feel stupid. this app teach calculus for a living and this app can assure you, i is stupid, not you.