Question

The exact value of cos 13pi/8 is

Answer 1

Answer:

answer is : Cos(13pi/8) = 0.3826  

Step-by-step explanation:

We have, Cos (13pi/8)

Since 13pi/8 can be shown as 3pi/2 < 13pi/8 < 2pi

Hence 13pi/8 lies on fourth quadrant.

In fourth quadrant cosine will be positive.

Cos (13pi/8) = cos(3pi/2 + pi/8)

applying formula cos(A+B) = cos A cosB - sinAsinB

i.e Cos(3pi/2 + pi/8) = cos(3pi/2)cos(pi/8) - sin(3pi/2)sin(pi/8)

∵ Remember cos(3pi/2) =0 , sin(3pi/2) = -1

     Cos(3pi/2 + pi/8) = 0 cos(pi/8) - (-1)sin(pi/8)

     Cos(3pi/2 + pi/8) = 0  + 0.3826

      Cos(3pi/2 + pi/8) = 0.3826

Hence we got Cos(13pi/8) = 0.3826                          

Answer 2

Answer:

sqt root 2-sqt root 2/4

Step-by-step explanation: