令t=x-2，然后使用等价无穷小sqrt(x-1)-1=sqrt(t+1)-1=0.5*tln(3-x)=ln(1-t)=-t；sin(pi*x)=sin(pi*t+2*pi)=sin(pi*t)=pi*t2^x-4=2^(2+t)-4=4*[2^t-1]=4*t*ln2得出结果=0.5*t*(-t)/[pi*t]/[4*t*ln2]=-1/8/pi/ln2
