x=sect,dx=sec^2 t tant 原式=S [sec^2 t tant/sect tant]dt=Ssectdt=S 1/cost dt=S cost/cos^2 t dt=S cost/(1-sin^2 t ]dt=S dsint/(1-sin^2 t ]={S dsint/(1-sin t ]+S dsint/(1+sin t ]}/2={ln(1+sin t)-ln(1-sint)}/2+ C={ln[(1+sin t)/(1-sint)]}/2+ C=ln|(1+sin t)/cost|+ C=ln|sect+tan t|+ C