Equations

In general we frame real life problems into some mathematical formula called equations in which we use constant and some variables(unknown)
Example: Ram is 12 years old.Ram 's age is 3/5 of Shyam's age what is Shyam's age.
Let Shyam's age is x then 3/5 of sham's age is 3*x/5
x*3/5=12
x=60/3=20
so Shyam's age is 20.

Linear equations

When there is a linear combination of unknown variables is there then eqn is linear
Example: 3x+ 2y+ 4z=9
Non Linear Equations: Equations in which non linear combination of the variable is there then eqn is non linear. When any 2 unknown variable multiplication present in the equation then it is the nonlinear equation.
Example: 2x+5yz=98 is a nonlinear equation because yz multiplication is there:
Quadratic equation in one variable: x^2 + 4*x + 2=0 is non linear because unknown variable x is being multiplied in term x^2
Quadratic equation in many variable: x^2 + y^2 +z^2 + w=0
cubic equation: x^3+3*x^2+2*x+5=0
exponential equations e^x+5=68