an为等差数列,a4+a7+a10=17,a4+a5+a6...+a14=77,ak=13,求k
问题描述：
an为等差数列,a4+a7+a10=17,a4+a5+a6...+a14=77,ak=13,求k
答
若m+n=p+q则am+an=ap+aq;a4+a10=2a7则3a7=17;a7=17/3;a4+a5+a6+···+a14=10*a9+a9=11a9=77 则a9=7；a9-a7=2d=7-17/3=4/3d=2/3;a1=5/3ak=a1+(k-1)d=5/3+(k-1)*2/3=13k=18希望采纳#^_^#祝学习进步!