Thread: Value of cos(2x-3pi/4).

Sin(x)= -2/(5^0.5), 3pi/2<x<2pi

then i have that cos(x)=+-(1-(-2/(5^0.5))^0.5=+-1/(5^0.5)
and tan(x)=((-2/(5^0.5))/(1/(5^0.5)=2

but what is the exaktly valus of this expression: cos(2x-3pi/4)?

Originally Posted by paulaa

Sin(x)= -2/(5^0.5), 3pi/2<x<2pi

then i have that cos(x)=+-(1-(-2/(5^0.5))^0.5=+-1/(5^0.5)
and tan(x)=((-2/(5^0.5))/(1/(5^0.5)=2

but what is the exaktly valus of this expression: cos(2x-3pi/4)?

Get cos(2x-3pi/4) in terms of cos(x) using compound angle and double angle formulae. Then substitute the information you've been given.