a为锐角,且sin(a-π/6)=1/3,则cosa=?
问题描述：
a为锐角,且sin(a-π/6)=1/3,则cosa=?
答
sin(a -π/6 )=1/3,sinacosπ/6-cosasinπ/6=1/3 √3sina/2-cosa/2=1/3 √3sina-cosa=2/3 √3sina=coaa+2/3 两边平方 3(sina)^2=(cosa)^2+4cosa/3+4/9 3-3(cosa)^2=(cosa)^2+4cosa/3+4/9 令x=cosa 4x^2+4x/3-23/9=0 ...