设等差数列首项为a1,公差为d. 2an=2a1+2(n-1)d ap+aq=a1+(p-1)d+a1+(q-1)d=2a1+(p+q-2)d=2a1+(2n-2)d=2a1+2(n-1)d=2an 2an=ap+aq
