Custom functions take the form: dependent variable = expression. The expression (entered in the Custom Function tab) may contain constants, operators, functions and arguments (x in 2D functions, x and y in 3D functions, and x, y, and z in ternary graphs). For example:

y = x^2+x

z = 2*x+3*y+12

z = 2*sin(2*sqrt(x^2+y^2))/sqrt(x^2)

v = 5*x - y^3 + 1/z