Out of the following equations which one is not a quadratic equation? (A) x² + 4x = 11 + x² (B) x² = 4x (C) 5x² = 90 (D) 2x - x² = x² + 5

Answer: (A) \(x^2 + 4x = 11 + x^2\)

Simplify each equation and look at the highest power of \(x\):

• (A) \(x^2 + 4x = 11 + x^2\) gives \(4x - 11 = 0\). The \(x^2\) terms cancel, so the degree is 1. This is a linear equation, not quadratic.
• (B) \(x^2 = 4x\) gives \(x^2 - 4x = 0\): degree 2, quadratic.
• (C) \(5x^2 = 90\) gives \(5x^2 - 90 = 0\): degree 2, quadratic.
• (D) \(2x - x^2 = x^2 + 5\) gives \(2x^2 - 2x + 5 = 0\): degree 2, quadratic.

\(\therefore\) The correct option is (A).