Question

这几道题怎么做？求助！！！

Answer 1

9.这是一个首项为4.9，公差为9.8的等差数列，求的是前10项的和：

a1 = 4.9，d = 9.8，则a10 = 4.9+9.8*9 = 93.1.

    则物体下降的总高度，即前10项的和为：(4.9+93.1)*10/2 = 490 m;

10.这是求首项为21，公差为1的等差数列的前20项的和：

a1 = 21， d = 1， 则a20 = 21 +1*19 = 40.

    则瓦片的总和为 (21+40)*20/2 = 510;

11. 3b - 6a = c;

      a+b+c = 15;

      a-b = b-c;

解上面的方程组，可得 a =1, b = 5, c =9;

B组

1. S(20) = -380, d = -2; 
   a20 = a1 + 19d = a1 - 38;————(1)

   S(20) = (a1 + a20)*20 / 2;————(2)

   联立解(1)(2)，得a1 = 0, a20 = -38;

2. S5 = 35, a11 = 31;

   S5 = (a1 + a5)*5/2;

   a5 = a1 + 4d;

   以上两式得：S5 = 5(a1 + 2d) = 5 * a3 = 35;

   解得 a3 = 7;

   a11 = a1 + 10d = 31;

   又：a11 - a3 = 8d = 31 - 7 = 24;

   解得： d = 3;

   a3 = a1 + 2d = 7;

   解得 a1 = 1.

   综上：a1 = 7, d = 3.

辛苦手打，望采纳。