Question

How do I find the value of cos (11pi/4)?

Answer 1

`cos((11pi)/4)=-(sqrt(2))/2`

Explanation:

First we have to notice, that `(11pi)/4 >2pi`, so we can use this to reduce the angle, because `cos(2pi+x)=cos(x)`

After this reduction we have to calculate `cos((3pi)/4)`. To do this we use the reduction formula:

`cos(pi-pi/4)=-cos(pi/4)=-(sqrt(2))/2`