等差数列an中,a1,a2,a3,a4成等比,s5=30,求an
问题描述:
等差数列an中,a1,a2,a3,a4成等比,s5=30,求an
答
S5=1/2(a1+a1+4d)*5 化简得a1+2d=6
a1,a2,a3,a4成等比,即(a1+d)(a1+2d)=a1*(a1+3d) 化简得2d^2=0
所以an=6 ,d=0
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天啊,我算错了么?!