∵b=a-(a+b)
cosb=cos[a-(a+b)].
=cosacos(a+b)+sinasin(a+b).
∵cosa=1/7,
∴sina=√1-cos^2a)=4√3/7
sin(a+b)=5√3/14,
cos(a+b)=√[1-sin^2(a+b)]
=√[1-(5√3/14)^2]
=11/14.
cosb=(1/7)*(11/14)+(4√3/7)*(5√3/14,)
cosb=71/98..
sin^2(20°)+sin80°*sin40°=sin^2(20°)+(-1/2)[cos(80°+40)-cos(80°-40)].
=sin^2(20°)-(1/2)cos120+(1/2)co40°.
=sin^2(20°)+1/4+cos^2(20°)-1/2.
=sin^(20)+cos^(20)+1/4-1/2
=1-1/2+1/4.
=3/4.
sin26°*sin16°+cos42/cos64°*cos74°+sin48°=?
题目中 cos74°是在分子上还是在分母上,请说清楚.