1 求sin10sin50sin70的值 2：若tan(π|4-α）=3，则cotα等于

1 .sin10sin50sin70=cos10sin10sin50sin70/cos10
=[1/2(sin20cos20cos40)]/cos10
=(1/4sin40cos400/cos10
=(1/8sin80)/cos10
=1/8

2 .tan-α=tan[(π|4-α）-π|4]=[tan(π|4-α）-tan(π|4）]/1+tan(π|4-α)
=1/2
所以tanα=-1/2

cotα等于1/tanα=-2

1、sin10sin50sin70=cos80`cos40`cos20`=sin20`cos20`cos40`cos80`/sin20`
=(1/2)sin40`cos40`cos80`/sin20`=(1/4)sin80`cos80`/sin20`=(1/8)sin160`/sin20`=(1/8)sin(180`-20`)/sin20`=(1/8)sin20`/sin20`=1/8

2、tan(π/4-α）=3=(1-tana)/(1+tana),tana=-1/2
cota=-2