设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求...设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求a3和s5 2.求公差范围 3.求证s8大于等于64
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![设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求...设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求a3和s5 2.求公差范围 3.求证s8大于等于64](/uploads/image/z/5559294-30-4.jpg?t=%E8%AE%BEsn%E4%B8%BA%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97an%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E5%B7%B2%E7%9F%A5a1%2Ba2%2Ba6%3D15+s7%E5%A4%A7%E4%BA%8E%E7%AD%89%E4%BA%8E49+1.%E6%B1%82...%E8%AE%BEsn%E4%B8%BA%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97an%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E5%B7%B2%E7%9F%A5a1%2Ba2%2Ba6%3D15+s7%E5%A4%A7%E4%BA%8E%E7%AD%89%E4%BA%8E49+1.%E6%B1%82a3%E5%92%8Cs5+2.%E6%B1%82%E5%85%AC%E5%B7%AE%E8%8C%83%E5%9B%B4+3.%E6%B1%82%E8%AF%81s8%E5%A4%A7%E4%BA%8E%E7%AD%89%E4%BA%8E64)
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设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求...设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求a3和s5 2.求公差范围 3.求证s8大于等于64
设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求...
设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求a3和s5 2.求公差范围 3.求证s8大于等于64
设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求...设sn为等差数列an的前n项和已知a1+a2+a6=15 s7大于等于49 1.求a3和s5 2.求公差范围 3.求证s8大于等于64
设公差为d
1.a1+a2+a6=15 即3a1+6d=15,a3=a1+2d=5,s5=5a1+10d=25
2.s7大于等于49,即7a1+21d≥49,a1+3d≥7,因为a1+2d=5,所以,d≥2
3.s8=s7+a1+7d≥49+5+5d≥64
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