第2个回答 2012-01-28
合积化差公式
sina*sinb=-1/2[cos(a+b)-cos(a-b)]
所以sin10°sin50°sin70°
=-1/2[cos60-cos(-40)]*sin70
=-1/4sin70+1/2cos40*sin70
=-1/4sin70+1/2sin50*sin70
=-1/4sin70+1/2*(-1/2)[cos120-cos(-20)]
=-1/4sin70+1/8+1/4cos20
=-1/4sin70+1/8+1/4sin70
=1/8
=0.125