Se deben conocer las siguientes formulas :
cos(a + b) = cos(a)cos(b) - sin(a)sin(b)
cos(a - b) = cos(a)cos(b) + sin(a)sin(b)
sin(a + b) = sin(a)cos(b) + cos(a)sin(b)
sin(a - b) = sin(a)cos(b) - cos(a)sin(b)
tan(a) = sin(a) / cos(a)
cotangente = 1 / tan = cos / sin
Partimos de uno de los miembros de la igualdad y tratamos de transformarlo a la expresion que aparece en el otro miembro
<img src="https://tex.z-dn.net/?f=%5Cfrac%7Bcos%28a%2Bb%29%20%2B%20cos%28a-b%29%7D%7Bsin%28a%2Bb%29%20%2B%20sin%28a-b%29%7D%3D%5Cfrac%7Bcos%28a%29cos%28b%29-sin%28a%29sin%28b%29%2Bcos%28a%29cos%28b%29%2Bsin%28a%29sin%28b%29%7D%7Bsin%28a%29cos%28b%29%2Bcos%28a%29sin%28b%29%2Bsin%28a%29cos%28b%29-cos%28a%29sin%28b%29%7D" />
Haciendo las operaciones queda demostrada :
<img src="https://tex.z-dn.net/?f=%5Cfrac%7B2cos%28a%29cos%28b%29%7D%7B2sin%28a%29cos%28b%29%7D%3D%5Cfrac%7B2%7D%7B2%7D%5Ccdot%5Cfrac%7Bcos%28a%29%7D%7Bsin%28a%29%7D%5Ccdot%5Cfrac%7Bcos%28b%29%7D%7Bcos%28b%29%7D%3D1%5Ccdot%5Cfrac%7Bcos%28a%29%7D%7Bsin%28a%29%7D%5Ccdot1%3D%5Cfrac%7Bcos%28a%29%7D%7Bsin%28a%29%7D%3D%5Cfrac%7B1%7D%7Btan%28a%29%7D" />
Suerte.