The value of ab if 3a + 5b = 15 and 9a² + 25b² = 75 is :

1. 4

2. 5

3. 6

4. 8

If l + m − n = 9 and l² + m² + n² = 31, then mn + nl − lm is :

1. -25

2. 25

3. -2

4. -5

Assertion (A): (26)³ + (−15)³ + (−11)³ = 3 × 26 × 15 × 11.

Reason (R): If x + y + z = 0, then x³ + y³ + z³ = 3xyz

1. A is true, R is false

2. A is false, R is true

3. Both A and R are true

4. Both A and R are false