Answer:
cos(a + b) =
Step-by-step explanation:
cos(a + b) = cos(a).cos(b) - sin(a).sin(b) [Identity]
cos(a) =
cos(b) =
Since, terminal side of angle 'a' lies in quadrant 3, sine of angle 'a' will be negative.
sin(a) = [Since, sin(a) =
]
=
=
Similarly, terminal side of angle 'b' lies in quadrant 2, sine of angle 'b' will be negative.
sin(b) =
=
=
By substituting these values in the identity,
cos(a + b) =
=
=
=
Therefore, cos(a + b) =