2cos70度+sin80度+cot70度*cos80
=2cos70+sin80+cos70cos80/sin70
=2cos70+(sin80sin70+cos80cos70)/sin70
=2cos70+cos10/sin70
=(2sin70cos70+cos10)/sin70
=(sin140+sin80)/sin70
=(sin40+sin80)/sin70
=[sin(60-20)+sin(60+20)]/sin70
=2sin60cos20/sin70
=2sin60sin70/sin70
=2sin60=根号3
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