Trigonometric ratios of an acute angle of a right-angled triangle. Proof of their existence (well defined); motivate the ratios whichever are defined at 0 and 90 degree. Values (with proofs) of the trigonometric ratios of 30, 45 and 60 degree. Relationships between the ratios. Proof and applications of the identity. Only simple identities to be given. Trigonometric ratios of complementary angles.