While networked software probably offers the best options for creating mathematical formulae, there are free apps that can help you visualise, solve and play with equations, including PhotoMath, My Script Calculator and Desmos.
PhotoMath – photographs a typed (not handwritten) mathematical equation, solves it, and provides you with the answer and shows its working. This app is reportedly improving all the time, and can handle an increasing number of equation types. Great for helping you understand how an equation is solved. Not so great for solving all your practice problems because you won’t have it in exams!
My Script Calculator – converts equations you write on the screen into a typed equation and then solves it and gives you the answer and the results can be copied and exported to other apps.
Desmos – Engage visually with mathematical concepts with this graphical calculator app (also available as a website). Visually engage with mathematics, plot functions, create tables, add sliders, animate graphs and solve equations.