直角三角形:
- 固定底边长度为1,则sin、cos:
公式:
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
- 固定临边长度为1,则tan、sec:
公式:
tan^2 + 1 = sec^2
- 固定对边长度为1,则cot、csc:
公式:
1+cot^2 = csc^2