设y=(1+x)m=a0+a1x+a2x2+…+amxm,y′=m(1+x)m-1=a1+2a2x+3a3x2+…+mamxm-1,令x=1,得2m-1m=a1+2a2+3a3+…+mam=192=25×6.解得m=6.∴a3=C63=20. 1 20 + 2 20 +…+ n 20 = n(n+1) 40 , n(n+1) 40 ≥ 3 4 解得n>4.正整数n的最小值为:5.故答案为:5.