所求为斜率k=tanθtan(θ-45°)=3，或者tan(θ+135°)=3∴tan(θ-45°) = (tanθ - tan45°) / (1 + tanθ×tan45°）= （k-1) / (1+k)或：tan(θ+135°) = (tanθ + tan135°) / (1 - tanθ×tan135°）= （k-1) / (1+k)
