Trig
posted by Don on .
Eight Fundamental Identities
Sec thada  Cos thada
How do I do this?

There is no equal sign so no identity.
sec t = 1/cos t
by definition if that helps 
sec ( theta ) = 1 / cos ( theta )
sec ( theta )  cos( theta ) =
[ 1 / cos ( theta ) ]  cos ( theta ) =
[ 1  cos ^ 2 ( theta )] / cos ( theta ) =
sin ^ 2 ( theta) / cos ( theta ) =
sin ( theta ) * sin ( theta ) / cos ( theta ) =
[ sin ( theta ) / cos ( theta ) ] * sin ( theta ) =
tan ( theta ) * sin ( theta )