1*4分之1+4*7分之1+7*10分之1+10*13分之1+13*16分之1
=(1/3)*(1-1/4+1/4-1/7+1/7-1/10+1/10-1/13+1/13-1/16)
=(1/3)*(1-1/16)
=(1/3)*15/16
=5/16