用诱导公式：
⑴sin(-1370°)
=sin(360°*4-1370°）
=sin70°>0.
⑵cos(25π / 7)
=cos[(25/7-4)π
=cos(-3/7)π
=cos(3π/7)>0.
⑶ tan(- 16π/5)
=tan(-16/5+3)π
=tan(-π/3)