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（4）原式=(0-1)/(0-0+4)=-1/4（5）原式=lim(x->3) (x+13-4x-4)/(x^2-9)[√(x+13)+2√(x+1)]=lim(x->3) (-3x+9)/(x+3)(x-3)[√(x+13)+2√(x+1)]=lim(x->3) -3/(x+3)[√(x+13)+2√(x+1)]=-3/(3+3)[√(3+13)+2√(3+1)]=-1/16
