1.

If 10tanA.tanB = 9, what is the value of cos(A – B)/cos(A + B)?1). 12). 03). 194). 9/10

Answer»

We know, COS(A – B) = cosAcosB + sinAsinB

And, cos(A + B) = cosAcosB – sinAsinB

Given, tanAtanB = 9/10

cos(A – B)/cos(A + B)

⇒ (cosAcosB + sinAsinB)/(cosAcosB – sinAsinB)

⇒ (1 + tanAtanB)/(1 – tanAtanB)

⇒ (1+ 9/10)/(1 – 9/10)

19

∴ cos (A – B)/cos (A + B) = 19


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