Question

Pls help in question 21 with full steps and explanation

Answer 1

$- cos(2 \alpha )$

$p( \sin( \alpha ) - q \div p \times \cos( \alpha ) ) \div p( \sin( \alpha ) + q \div p \times \cos( \alpha ) ) \\ q \div p = \cos( \alpha ) \div \sin( \alpha ) \\ \ { \sin( \alpha ) }^{2} - { \cos( \alpha ) }^{2} \div { \sin( \alpha ) }^{2} + { \cos( \alpha ) }^{2} \\ { \sin( \alpha ) }^{2} - { \cos( \alpha ) }^{2} = - \cos(2 \alpha )$