原式={75/[(5-x)*(x^2+5x+25)]-1/(5-x)}=[75-x^2-5x-25]/[(5-x)*(x^2+5x+25)] =[x^2+5x-50]/[(x-5)*(x^2+5x+25)]=[(x-5)(x+10)]/[(x-5)*(x^2+5x+25)] =(x+10)/(x^2+5x+25)带入x=5,可得原式=15/75=1/5
