Avem formulele: sin(a+b)=sin a •cos b+sin b •cos a
cos(a-b)=cos a•cos b+sin a• sin b
sin 75°=sin(30°+45°)=sin 30°•cos 45°+sin 45°•cos 30°=1/2•√2/2+√3/2•√2/2=√2/4+√6/4=(√2+√6)/4
cos 225°=cos( 270°-45°)=cos 270°•cos 45°+sin 270°•sin 45°=0•√2/2+(-1)•√2/2=0-√2/2=-√2/2
