Find the exact value of cos(11p/8)

Question

Find the exact value of cos(11p/8)
A) √(2+√2)/4
B) √(2-√2)/4
C) - √(2-√2)/4
D) -√(2+√2)/4

 

 

I think it is either c or b

 

 

 

To solve the equation 3sin(2x)=5cosx you should rewrite it as:

a) 3sinx+5cosx=0

b)cosx(6sinx-5)=0

c) cosx(3sinx-5)=0

 

 

explain? 

 

 

If 2tanx/1-tan^2x=1 then x can equal?

 

a) x=3pi/8 +n(pi)

b) x=pi/8 +n(pi)

c) x=5pi/8 + n(pi)

d) x=7pi/8 +n(pi)

 

check all that apply

 

 

 

Answer 1

11π/8 = (1/2)(11π/4)

 

cos 11π/8 = ±(√(1+cos(11π/4) / 2))

cos 11π/8 = ±(√(1+cos(3π/4) / 2))

cos 11π/8 = ±(√(1+(2√2)/2) / 2 )

cos 11π/8 = ±(√(1/2 + √2/4)

cos 11π/8 = ±(√(2 + √2) / 4)

cow 11π/8 = ±(√(√2 + √2) / 2)

 

11π/8 is in QIV,the cosine is positive.

 

B.