4/2+4*7/2*6+4*7*10/2*6*10+……一般项An=(4*7*10.(3n+1))/(2*6*10.(4n-2))A(n+1)/An=[(4*7*10.(3n+4))/(2*6*10.(4n+2))]/[(4*7*10.(3n+1))/(2*6*10.(4n-2))]=(3n+4)/(4n+2)趋于3/4