∫|1-x|dx [上限为5下限为0]
=∫（1-x）dx [上限为1下限为0]+∫（x-1）dx [上限为5下限为1]
=[x-x^2/2][上限为1下限为0]+[x^2/2-x] [上限为5下限为1]
=1-1/2+25/2-5-1/2+1
=17/2