题目

已知等差数列{an}的公差d=1,前n项和为Sn. (1)若1,a1,a3成等比数列,求a1; (2)若S5>a1a9,求a1的取值范围. 答案：解:(1)因为数列{an}的公差d=1,且1,a1,a3成等比数列, 所以=1×(a1+2),即-a1-2=0, 解得a1=-1或a1=2.(2)因为数列{an}的公差d=1,且S5>a1a9, 所以5a1+10>+8a1, 即+3a1-10<0, 解得-5<a1<2.Peter needs ______.A. three slices of bread B. three slices bread C. three slice of bread