先做不定积分令x=3tanu,则√(x^2+9)=3secu,dx=3sec²udu,∫ √(x^2+9)dx=∫ 3secu*3sec²udu=9∫ sec³udu下面计算：∫ sec³udu=∫ secud(tanu)=secutanu-∫ tan²usecudu=secutanu-∫ (sec
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