1/(5×8)+1/(8×11)+1/(11×14)+1/(14×17)+1/(17×20)
=(1/3)(1/5-1/8+1/8-1/11+1/11-1/14+1/14-1/17+1/17-1/20)
=(1/3)(1/5-1/20)
=(1/3)(3/20)
=1/20