sinθ-y cosθ＝√(1+y^2)·{1/√(1+y^2)·sinθ-y/√(1+y^2)·cosθ}假设sin∅＝y/√(1+y^2)则上式＝√(1+y^2)·{cos∅sinθ-sin∅cosθ)＝√(1+y^2)·sin(θ-∅)
