（tan2X—tanX)/(1+tan2X tanX)
=tan(2X—X)
=tanX
=2
sin(X—Y)cosY+cos(X—Y)sinY
=sin(X—Y+Y)
=sinX
cos80°sin50°—sin40°sin80°
=cos80°(sin90°-40°)—sin40°sin80°
=cos80°cos40°—sin80°sin40°
=cos(80°+40°)
=cos120°
=-1/2
X,Y为锐角,且sinX=(12/13),sinY=(3/5)
cosX=5/13,cosY=4/5
sin（X+Y)
=sinXcosY+cosXsinY
=12/13*4/5+5/13*3/5
=48/65+15/65
=63/65
sin15°cos15°
=1/2sin30°
=1/2*1/2
=1/4
2（cos^2 ）* （π/8) —1
=cos(2*π/8)
=cos(2*π/8)
=cosπ/4
=√2/2
cosX=（1/5）,（3π/2）