Sundry notes is really powerful.
It has a bit of a learning curve, but lots of flexible features.
You can even record and put the sound recording right there in the middle of your note.
I found a solution. I realize this is an ancient thread, but hopefully it'll help others that stumble upon this.
Copy this radical sign √
If you're having difficulty, search for one on google and copy one off another page.
Now go to settings - general - keyboard. At the bottom is an option to add a shortcut. Paste the radical symbol into the "phrase" section. And put whatever you like in the "shortcut" section. For instance, I put ^^ in under shortcut. Now when i type ^^ on my phone/Ipad it auto corrects to the radical. You could use this method for other math symbols as well.