x趋进于1时、（1减去x的4次方）分之4减去（1减去x的3次方）分之3的极限

lim[x→1][4/(1-x^4)-3/(1-x^3)]
=lim[x→1][4(1-x^3)-3(1-x^4)]/[(1-x^4)(1-x^3)]
=lim[x→1](1-4x^3+3x^4)/(1-x^3-x^4+x^7)
=lim[x→1](-12x^2+12x^3)/(-3x^2-4x^3+7x^6)
=lim[x→1](-24x+36x^2)/(-6x-12x^2+42x^5)
=12/24
=1/2