Quadratic Equations

A quadratic equation is an equation of degree 2, which means the highest power of the variable is 2.
Step 1: Option (1) is a quadratic equation as it simplifies to a degree 2 equation.
Step 2: Option (2) simplifies to a quadratic equation upon expansion.
Step 3: Option (3) is also a quadratic equation.
Step 4: Option (4) is a cubic equation, as the highest degree is 3.
Thus, the correct answer is option (4), as it is not a quadratic equation.