tan70°*cos10°*(√3 *tan20° -1) =tan70°*cos10°*(tan60 °*tan20° -1) =tan70°*cos10°*[(sin60 °*sin20°/cos60°*cos20°)-1] =tan70°*cos10°*(sin60 °*sin20°-cos60°*cos20°)/(cos60 °*cos20°) =tan70°*cos10°*[-cos(60°+20°)]/(cos60 °*cos20°) =-tan70°*cos10°*cos80°/(cos60 °*cos20°) =-tan70°*cos10°*sin10°/(cos60 °*cos20°) =-(sin70°/cos70°)*(1/2)*sin20°/(cos60 °*cos20°) =-(cos20°/sin20°)*sin20°/(2cos60 °*cos20°) =-1/(2cos60 °) =-1/4