一堆钢管堆放6层,相邻两层数量相差1根,其中最上层4根,这堆钢管有多少更?He can climd up___wall.
问题描述：
一堆钢管堆放6层,相邻两层数量相差1根,其中最上层4根,这堆钢管有多少更?He can climd up___wall.
You__ very good at basketball in China.
改错：What can I do better of all
You can jump really highly
You can sing it very well---yes,I was
答
第一层为4根,每层差一根,所以第六层就是9根,利用等差数列求和公式可得：
Sn=[n(A1+An)]/2
Sn=[6*(4+9)]/2
Sn=39