ABC is an isosceles triangle right-angled at C. Then 2AC² is equal to :

1. BC²

2. AC²

3. AC² - BC²

4. AB²

Since, ABC is an isosceles triangle right-angled at C.

∴ ∠A = ∠B

∴ BC = AC [Sides opposite to equal angles are equal] ……..(1)

In right-angled △ ABC,

By pythagoras theorem,

⇒ (Hypotenuse)² = (Perpendicular)² + (Base)²

⇒ AB² = AC² + BC²

⇒ AB² = AC² + AC² [From equation (1)]

⇒ AB² = 2AC².

Hence, Option 4 is the correct option.