斜率等于-1; 倾斜角135度
K= (sin70 - sin20)/(cos70 - cos20)
而 sin(70)=cos(90-70)=cos(20); cos(70)=sin(90-20))=sin(20)
故 斜率 k = (cos20-sin20)/(sin20 - cos20) = -1
倾斜角= arctan(k) = arctan(-1) = 135°