3/(x-1)²-3x/(x-1)²=(3-3x)/(x-1)²=3(1-x)/(x-1)²
因为(x-1)²=(1-x)²
所以：3(1-x)/(x-1)²=3(1-x)/(1-x)²=3/(1-x)
所以,原式=3/(1-x)