Is this answer enough The middle one is not proved , please prove it - Maths - Introduction to Trigonometry

Question

Is this answer enough? The middle one is not proved , please
prove it

Varun Rawat · Accepted Answer

Refer the following link :

https://www meritnation
com/cbse/class10/ncert-solutions/math/math/introduction-to-trigonometry/page193-exercise-8
4-qno5/9_135_2020

Is this answer enough? The middle one is not proved , please prove it.