a^2=（sin12°+cos12°）^2=1+2sin12°cos12°=1+sin24°
b^2=2(sin56°)^2=1-cos112°=1+sin22°
所以a>b
此题所用公式sin^2(A)+cos^2(A)=1
cos(2α)=cos^2(α)-sin^2(α)=1- 2sin^2(α)=2cos^2(α)-1
sin(2α)=2sin(α)cos(α)