Which of the following is not a quadratic equation?

1. 3x - x² = x² + 5

2. (x + 2)² = 2(x² - 5)

3. $(\sqrt{2}x + 3)^{2} = 2x^{2} + 6$

4. (x - 1)² = 3x² + x - 2

Solving,

$\Rightarrow (\sqrt{2}x + 3)^{2} = 2x^{2} + 6 \\[1em] \Rightarrow 2x^2 + 6\sqrt{2}x + 9 = 2x^{2} + 6 \\[1em] \Rightarrow 2x^2 - 2x^2 + 6\sqrt{2}x + 9 - 6 = 0 \\[1em] \Rightarrow 6\sqrt{2}x + 3 = 0 \\[1em]$

The highest power of equation is 1 and it's not equivalent to ax² + bx + c = 0 . Thus it is a linear equation.

Hence, option 3 is the correct option.