How can we know what cos(-75) is? We need to prove it using the sum and difference formula. We also need to use special triangles. how? I've tried doing cos(a-b) but I did cos(-30)cos(-75)

$$\cos(-75^\circ)=\cos(75^\circ)=\cos(45^\circ+30^\circ)=\cos(45^\circ)\cos(30^\circ)-\sin(45^\circ)\sin(30^\circ)$$
Can you take it from here?
