1.cos36`cos72`
=2cos36cos72sin36/2sin36
=2sin72cos72/4sin36
=sin144/4sin36
=1/4
tan5`-tan85
=tan5°-cot5°
=[(sin5°)^2-(cos5°)^2]/sin5°cos5°
=-2cos10°/sin10°
cos70°/(1+sin70°)=sin20°/(1+cos20°)
=2sin10°cos10°/2(cos10°)^2
=tan10°
(tan5°-tan85)[cos70°/(1+sin70°)]=-2