Quad Solver is in my (not-so-humble) option the single best application in the App Store for dealing with quadratic equations.
Here's the description I wrote for iTunes:
Quad Solver is the best, full featured iOS application for solving Quadratic Equations and using the quadratic formula. It's easy to use, fast, and best of all: it's FREE!
Features of Quad Solver include:
- Support for imaginary roots (Complex Numbers)
- Solves the Discriminate
- Solves the Vertex
- Solves for Symmetry
- Graphing Help
- Advanced Help information
- Solves the Derivative
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