# If it is given that Sin A = Cos A then calculate Cos 2A?

+1 vote
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If it is given that Sin A = Cos A then calculate Cos 2A?
posted Jan 21, 2015

Sin A=Cos A
implies A=45 degress
Cos 2A= Cos 2(45)=Cos 90=0

Ans : 0

Use the the formula.
cos (A+B) = cos A cos B - Sin A Sin B

So in our case :
cos 2A = Cos (A + A)
= cos A cos A - sin A sin A
since: cos A = sin A
so
= cos^2 A - cos^2 A
= 0

+1 vote

Sin A = Cos A

Divide by Cos A gives us Tan A = 1 i.e. A is 45

so Cos 2A is Cos 90 which is zero

Use the the formula.
cos (A+B) = cos A cos B - Sin A Sin B

So in our case :
cos 2A = Cos (A + A)
*Sin A = CosA **
implies A=45 degress
Cos 2A= Cos 2(45)=Cos 90=0

so the right answer is zero(0)

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