Part a is not an identity.
If you try to establish: sin(t) / ( 1 + cos(t) ) = csc(t)
---> sin(x) / ( 1 + cos(t) ) · [ ( 1 - cos(t) ) / ( 1 - cos(t) )
---> [ sin(x) · ( 1 - cos(t) ) ] / [ ( 1 + cos(t) ) · ( 1 - cos(t) ) ]
---> [ sin(x) · ( 1 - cos(t) ) ] / [ 1 - cos2(t) ] since 1 - cos2(t) = sin2(t) --->
---> [ sin(x) · ( 1 - cos(t) ) ] / [ sin2(t) ]
---> ( 1 - cos(t) ) / sin(t)
---> 1 / sin(t) - cos(t) / sin(t)
---> csc(t) - cot(t) not just csc(t)
Part b: ( sec2(t) - 1 ) / sec2(t) = sin2(t)
---> ( sec2(t) - 1 ) / sec2(t)
---> sec2(t) / sec2(t) - 1 / sec2(t)
---> 1 - cos2(t)
---> sin2(t)
Part c: Write 1 / [ 1 + sin(x) ] without fractions:
---> 1 / [ 1 + sin(x) ] · [ ( 1 - sin(t) ) / ( 1 - sin(t) ) ]
---> [ 1 · ( 1 - sin(t) ) ] / [ ( 1 + sin(t) ) · ( 1 - sin(t) ) ]
---> [ 1 - sin(t) ] / [ 1 - sin2(t) ]
---> [ 1 - sin(t) ] / [ cos2(t) ]
---> 1 / cos2(t) - sin(t) / cos2(t)
---> sec2(t) - sin(t) / cos(t) · 1 / cos(t)
---> sec2(t) - tan(t) ·sec(t)