3.lim(x→0)[(tanx-sinx)/x^3]=lim(x→0)[sinx(1-cosx)/(x³cosx)]=lim(x→0){(sinx/x)*[sin(x/2)/(x/2)]²*[1/(2cosx)]}=[lim(x→0)(sinx/x)]*{lim(x→0)[sin(x/2)/(x/2)]}²*{lim(x→0)[1/(2cosx)]}=1...3. =3*lim(x→π)[sin(3(x-π))/(3(x-π))] (應(yīng)用誘導(dǎo)公式) 這一部我不懂lim(x→π)[sin(3(π+x-π))/(x-π)]=lim(x→π)[3*sin(3(π+x-π))/(3(x-π))] (分子分母同乘3，且x=π+x-π)=3*lim(x→π)[sin(3π+3(x-π))/(3(x-π))] =3*lim(x→π)[sin(3(x-π))/(3(x-π))] (應(yīng)用誘導(dǎo)公式,即sin(3π+3(x-π))=sin(3(x-π)))