上下乘√2/2原式=(sin70*√2/2-cos70*√2/2)/(sin70*√2/2+cos70*√2/2)=(sin70cos45-cos70sin45)/(sin70cos45+cos70sin45)=sin(70-45)/sin(70+45)=sin25/sin115=sin25/sin(90+25)=sin25/cos25=tan25