你好!原式= lim [√(3n+n²) - n] [√(3n+n²) + n] / [√(3n+n²) + n]=lim [(3n+n²) - n²] / [√(3n+n²) + n]=lim 3n / [√(3n+n²) + n]=lim 3 / [√(3/n +1) +1]= 3/2