If 4x² + y² = a and xy = b, find the value of 2x + y.

The expansion of (x - 3y)(x + 5y) is :

1. x² + 2xy + 15y²

2. x² + 2xy - 15y²

3. x² - 2xy + 15y²

4. x² - 2xy - 15y²

If a + b - c = 4 and a² + b² + c² = 14, the value of ab - bc - ca is :

1. 2

2. 1

3. 0.5

4. -0.5

$\Big(a - \dfrac{1}{2a}\Big)^2$ is equal to :

1. $\Big(a^2 + \dfrac{1}{4a^2} - 2\Big)$

2. $\Big(a^2 + \dfrac{1}{4a^2} + 2\Big)$

3. $\Big(a^2 + \dfrac{1}{4a^2} - 1\Big)$

4. $\Big(a^2 - \dfrac{1}{4a^2} - 2\Big)$