数列{an}为等差数列,且a5+a8+a13+a16=2000
a5+a16=a8+a13=a1+7d+a1+12d=2a1+19d=a1+a1+19d=a1+a20=1000
S20=10(a1+a20)=20*(a1+a20)/2=20*1000/2=10000