x=tant dx=sec²tdt √x²+1=sect 原式=∫{1/[(tant)^4sect]}sec²tdt =∫(cos³t/(sint)^4)dt sint=u costdt=du =∫[(1-u²)/u^4]du =(-1/3)/u³+1/u+c =(-1/3)(√(x²+1)/x)³+(√(x²+1)/x)³+c
