4. We describe locations on the Earth using latitude and longitude 0. Using these coordinates, we describe a location A on the Earth with A = (OA, 0A). Las Vegas, for example, has coordinates (36 N, 115 W), or (36, 115). Prove the distance formula cos(|AB|/R)=sin sin b + cos OA COS OB COS(0A - OB), where R is the radius of the Earth. Hint: Look at AABN where N is the North pole.