高一数学题求助,大家快来,明早要交~
设公差为d S9 =9a1+9(9-1)d/2 =18+36d=36 ===>d= 1/2 a3 = a1+2d =3 a9 =a1+8d =6 a3,a9,am成等比数列 ==>(a9)^2 =a3*am ===>am =12 am=a1+(m-1)d = 2+(m-1)/2 =12 ==>m=21
m＝21 解法： 等差数列的和公式Sn＝a1×n+n(n-1)d/2 所以S9＝2×9+9×8d/2＝36 解得d＝1/2 所以am＝2+(m-1)*(1/2)＝（1/2）(m+3) a3＝a1+2d=3 a9=a1+8d=6 又因为a3,a9,am成等比数列，所以a9^2=a1*am 即36=(3/2)(m+3) m＝21