Q20 of 25 Page 10

Using vectors, prove that cos(A – B) = cosA cosB + sinA sinB.

Let α and β be two unit vectors in the xy- plane.


And, α and β make angles A and B with the x-axis.


Therefore, The angle between α and β is (A-B)


Thus,




Then,



Where,



Since,



Hence, Proved


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