直角三角形:
sin^2 + cos^2 = 1
sin(x+y) = sin(x)cos(y)+cos(x)sin(y)
=> sin(2x) = 2sin(x)cos(x)
cos(x+y) = cos(x)cos(y)-sin(x)sin(y)
=> cos(2x) = cos(x)^2-sin(x)^2 = 2*cos(x)^2 - 1 = 1 - 2*sin(x)^2
=> cos(x)^2 = ( 1 + cos(2x) ) / 2
=> sin(x)^2 = ( 1 - cos(2x) ) / 2
tan^2 + 1 = sec^2
1+cot^2 = csc^2