Determine the value of cosine function if angle is 315 degrees .

We'll express in radians the equivalent value of 315
degrees.

315 degrees =
7pi/4

We'll write 7pi/4 = pi +
3pi/4

We'll apply cosine
function:

cos (7pi/4) = cos (pi +
3pi/4)

cos (pi + 3pi/4) = cos pi*cos (3pi/4) - sin
pi*sin(3pi/4)

sin pi = 0 and cos pi =
-1

cos (pi + 3pi/4) = - cos
3pi/4

cos 3pi/4 = cos (pi-pi/4) = cos pi*cos pi/4 + sin
pi*sin pi/4

cos 3pi/4 = - cos pi/4 => - cos 3pi/4 =
cos pi/4 = sqrt 2/2

cos (7pi/4) = sqrt
2/2

The value of cosine function if the angle
is 315 degrees is sqrt2/2.