a) 1 - sin^2a + ctg^2a sin^2a= cos^2a + cos^2a sin^2a= cos^2a(1 + sin^2a)= cos^2a
b) (tga cosa)^2 + (ctga sina)^2= (sin^2a/cos^2a cos^2a)^2 + (cos^2a/sin^2a sin^2a)^2= sin^2a + cos^2a= 1
a) 1 - sin^2a + ctg^2a sin^2a
= cos^2a + cos^2a sin^2a
= cos^2a(1 + sin^2a)
= cos^2a
b) (tga cosa)^2 + (ctga sina)^2
= (sin^2a/cos^2a cos^2a)^2 + (cos^2a/sin^2a sin^2a)^2
= sin^2a + cos^2a
= 1