Which of the following equations is quadratic?
x(x² + 1) = 0, 5(4x + 2) = 3, (x + 3)(x + 4) = 5

Solution:

A quadratic equation is an equation which has the highest degree of power 2.

Solving each equation one by one

1) x(x² + 1) = 0

⇒ x³ + x = 0 (Highest degree = 3)

2) 5(4x + 2) = 3

⇒ 20x + 10 = 3

⇒ 20x + 7 = 0 (Highest degree = 1)

3) (x + 3)(x + 4) = 5

⇒ x² + 4x + 3x + 12 = 5

⇒ x² + 7x + 12 = 5

⇒ x² + 7x + 7 = 0 (Highest degree = 2)

Therefore, the quadratic equation in the given set of equations is (x + 3)(x + 4) = 5

Which of the following equations is quadratic?
x(x² + 1) = 0, 5(4x + 2) = 3, (x + 3)(x + 4) = 5

Summary:

The quadratic equation is an equation having the highest degree = 2. So (x + 3)(x + 4) = 5 is a quadratic equation.