等比数列an前n项和为sn,且s4=3,s8=9,则a9+a10+a11+a12=?
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![等比数列an前n项和为sn,且s4=3,s8=9,则a9+a10+a11+a12=?](/uploads/image/z/15109806-30-6.jpg?t=%E7%AD%89%E6%AF%94%E6%95%B0%E5%88%97an%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BAsn%2C%E4%B8%94s4%3D3%2Cs8%3D9%2C%E5%88%99a9%2Ba10%2Ba11%2Ba12%3D%3F)
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等比数列an前n项和为sn,且s4=3,s8=9,则a9+a10+a11+a12=?
等比数列an前n项和为sn,且s4=3,s8=9,则a9+a10+a11+a12=?
等比数列an前n项和为sn,且s4=3,s8=9,则a9+a10+a11+a12=?
等比数列的话有a9+a10+a11+a12=q^4(a5+a6+a7+a8)=q^4*(S8-S4)=q^4*6
有a5+a6+a7+a8=q^4(a1+a2+a3+a4)=q^4*3=6,得q^4=2,所以a9+a10+a11+a12=12.