Prove by vector method that `cos(A+B) = cosAcosB - sinAsinB`

Prove by vector method that cos(A+B)cos A cos B-sin A sin B

Prove by vector method that sin(A-B)=sin A cos B-cos A sin B and sin(A+B)=sin A cos B+cos A sin B

Using vector method prove that cos(A-B)=cosAcosB+sinAsinB

In a triangleABC , prove by vector method that cos2A+cos2B+cos2Cge(-3)/(2) .

Prove by vector method that the area of /_\ABCis (a^2sinBsinC)/(2sinA) where symbols have their usual meanings.

Prove by vector methods that sin(alpha+beta)=sin alpha cos beta+cos alpha sin beta

Prove by vector method that the sum of the square of the diagonals of a parallelogram is equal to the sum of the squares of its sides.

prove by vector method that the sum of the squares of the diagonals of a parallelogram is equal to the sum of the squares of its sides.

For any A,B in R, prove that cos(A-B)=cos A cos B+sin A sin B

Prove that formula cos(A-B)=cos A*cos B+sin A*sin Bwith the help of the formula sin(A+B)=sin A*cos B+cos A*sin B