问已知等差数列的项数为2k+1(k属于整数），其奇数项之和与偶数项之和的比为？

a3=a1+2d Sn=na1+n(n-1)d/2 a4=a2+2d S1=a1+a3+...+ak+1=a1(k+1)+k(k+1)d=a1k+a1+k^2d+kd=(a1+kd)k+(a1+kd)=(a1+kd)(k+1) S2=a2+a4+...ak=a2k+k(k-1)2d/2=a1k+dk+k^2d-kd=a1k+k^2d=(a1+kd)k S1/S2=(k+1)/k