Check whether the following are quadratic equations:

Let us solve the above equation by equating it to zero,

x² + 1 + x³ = 0 × x

x² + 1 + x³ = 0

Since on solving the equation we do get the third power of x along with second power which proves that the following set of equation is not quadratic. A quadratic equation contain the highest power of x which is 2.