cos310°*tan(-108°)=cos(360-310)*[-tan(108°.
=cos50°*[-tan(180-72).
=cos50°*[-(-tan72°].
=cos50°*tan72°.>0
cos5π/4cos4π/5tan11π/6= cos(4π+π)/4*1/5*tan(π+5π/6).
=cosπ/4/5tan(5π/6).
=(-5/4)*!an(5π/6)
=(-5/4)tan(π-π/6).
=(-5/4)(-tanπ/6).
=(5/4)tan(π/6)>0