分子分母同除以cos^3a(sin^3 a+2cos^ 3 a)/(sin^3 a-2cos^3 a)=(sin³a/cos³a+2)/(sin³a/cos³a-2)=(tan³a+2)/(tan³a-2)=[(1/3)³+2]/[(1/3)³-2]=(1+2*27)/(1-2*27)=-55/53...