Hey i have this equation;

a² + b² - 2abCos Ѳ = c² + d² - 2cdCos ø

and i need to show that dѲ/dø = cd sinø / ab sinѲ

im not quite sure how to do this

i think that the 1st one goes to 2a + 2b + 2absinѲ

and the 2nd one goes to 2c + 2d + 2cdsinø

but im stuck from there