・Dual-line display - shows the input and result at the same time
・Percentage and backspace button
・18 color themes
・Multi-line input for really long equations
・Automatic scientific formatting
・Swipe left on the display to backspace
・Swipe right on the display to clear
・Tap & hold on the result to copy
・Tap & hold on the input to move the cursor, copy, paste
The only difference is that the Free version has ads.
No, you only have to pay once and then you can download it again for free onto all of your devices.
The app is universal and works with both the iPhone and the iPad.
You can also share your purchase with others in your family if you have Family Sharing enabled on your account.
For more information about Downloading Previous Purchases and Family Sharing, please visit Apple at https://support.apple.com/billing
The Paid version of the app is a totally separate app, so look for a new icon on your device without the word Lite on it.
Start using the Paid version from now on, and delete the Free version from your device so that you do not open it by mistake.
Here is what the two different icons look like for reference: