we get repeatative values of sin , cos and tan etc after 360 degree. so if at 90 degree, some value is changes from say cos to sin then it will be repeatative after every 360 degree. now see the graph. sin and cos are identical only there is phase difference of 90 degree. graph is exactly same if x- axis is moves by 90 degree. similarly other value u say are also interchangable. this is graph proof.