The longest road that can be placed in a rectangular box = 20 cm and the sum of its length breadth and height is 30 cm. Find the total surface area of the box.

If $a + \dfrac{1}{a} = 2.5 \text{ and } a - \dfrac{1}{a} = 1.5$, the value of $\Big(a + \dfrac{1}{a}\Big)^2 - \Big(a - \dfrac{1}{a}\Big)^2$ is :

1. 4

2. 2

3. 1

4. 8.5

(x + y - z)(x - y + z) is equal to :

1. x² - y² - z² - 2yz

2. x² - y² - z² + 2yz

3. x² - y² + z² - 2yz

4. x² - y² + z² + 2yz

If (3x - 4y)² = 9x² + axy + 16y²; the value of a is :

1. -24

2. 24

3. -12

4. 12