Mark the correct alternative in the following:

The value of is

If sin α – sin β = a and cos α + cos β = b, then write the value of cos (α + β).

If and then write the value of α + β lying in the interval (0, π/2).

Mark the correct alternative in the following:

If A + B + C = π, then sec A (cos B cos C – sin B sin C) is equal to

Mark the correct alternative in the following:

tan 20^o + tan 40^o + √3 tan 20^o tan 40^o is equal to