(2sin80-cos70)/cos20=[2sin(60+20)-sin20]/cos20=[2(sin60cos20+cos60sin20)-sin20]/cos20=(2sin60cos20+2cos60sin20-sin20)/cos20因为cos60=1/2所以,原式=(2sin60cos20+2×1/2sin20-sin20)/cos20=(2sin60cos20+sin...