Question

If 3 tan a tan b = 1 , prove that 2 cos ( A+B ) = cos ( A- B).​

Answer 1

Step-by-step explanation:

Given that,

3tan A tanB=1

=1/tanAtanB=3

=cosA cosB/sinA sinB=3/1

=cosAcosB+sinA sinB/cosA cosB-sinA sinB=3+1/3-1

=cos(A-B)/cos(A+B)=4/2

=2cos(A-B)=cos(A+B)

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