sin105度等于多少？

一、sin105度等于多少？

sin105=sin(90+15)=sin15

二、1/2cos30°+√2/2cos45°-tan60°*cos60°

1

1/2cos30°+√2/2cos45°-tan60°*cos60°

=sin30°cos30°+sin45°cos45°-sin60°

=1/2×√3/2+1/2-√3/2

=4-√3/4

三、tan2α=1/3求tanα=？

tan2α=1/3

2tana/(1-tana^2)=1/3

6tana=1-tana^2

tana^2+6tana-1=0

tana=[-6±√40]/2 =-3±√10

四、tan70°COS10°（根号3tan20°-1）=

tan70×cos10×(√3tan20-1)

=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