Trig identity is equations that are true for Right Angled Triangles. D1‎: ‎sin 2A+ cos 2A=1 D2‎: ‎1+ tan 2A= sec 2A D3‎: ‎1+ cot 2A= csc 2A Hence trigonometric identities are prove by  above D1,D2 and D3. Similarly also using reciprocal identity. Trigonometric Identity