theorem Th30: :: SINCOS10:30

( sin . (- (PI / 4)) = - (1 / (sqrt 2)) & cos . (- (PI / 4)) = 1 / (sqrt 2) & sin . ((3 / 4) * PI) = 1 / (sqrt 2) & cos . ((3 / 4) * PI) = - (1 / (sqrt 2)) )