p(-3,4)为角a终边上的一点
即：
x=-3,y=4,r=√{(-3)^2+4^2} = 5
sinα = y/r = 4/5
cosα = x/r= -3/5
tanα = y/x = -4/3
cotα = x/y = -3/4
secα = r/x = -5/3
cscα = r/y = 5/4