An equation is quadratic if the highest power of the variable in it is 2.
\(1. \\
x(x^2+1)=0 \\
x^3+x=0 \\
\hbox{the highest power: 3} \\
\hbox{not quadratic} \\ \\
2. \\
5(4x+2)=3 \\
20x+10=3 \\
20x+7=0 \\
\hbox{the highest power: 1} \\
\hbox{not quadratic} \\ \\
3. \\
(x+3)(x+4)=5 \\
x^2+4x+3x+12=5 \\
x^2+7x+7=0 \\
\hbox{the highest power: 2} \\
\hbox{quadratic}\)
(x+3)(x+4)=5 is a quadratic equation.
