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
