2/(3n-2)(3n+1)=(2/3)*[1/(3n-2)-1/(3n+1)]
所以原式=(2/3)*[1-1/4+1/4-1/7+1/7-1/10+1/10-1/13+.+1/(3n-2)-1/(3n+1)]
=(2/3)*[1-1/(3n+2)]
=(6n+2)/(9n+6)