If a is a positive and $a^2 +\dfrac{1}{a^2}= 18$; then the value of $a -\dfrac{1}{a}$ is:

1. 4

2. 16

3. 20

4. 2√5

(x + y)(x - y)(x² + y²)(x⁴ + y⁴) is equal to:

1. x⁴ + y⁴

2. x⁸ + y⁸

3. x⁸ - y⁸

4. 2x⁶y⁶

(x + 3) (x + 3) - (x - 2) (x - 2) is equal to:

1. 10x + 5

2. 10x - 5

3. 5 - 10x

4. none of these

If 5a = 30² - 25², the value of a is:

1. 5

2. 11

3. 55

4. none of these