紧急救命!等差数列!{An}中,【1】a2+a3+a11+a12=40,求a7;【2】a2+a3+a4+a5=34,a2×a5=52,求公差d另一道:a1=1/3,a2+a5=4,An=33,求n第3道:S20=120,则a2+a19=?第4道:a4=0.8,An=2.2,求a51+a52...+a80=?第5道
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![紧急救命!等差数列!{An}中,【1】a2+a3+a11+a12=40,求a7;【2】a2+a3+a4+a5=34,a2×a5=52,求公差d另一道:a1=1/3,a2+a5=4,An=33,求n第3道:S20=120,则a2+a19=?第4道:a4=0.8,An=2.2,求a51+a52...+a80=?第5道](/uploads/image/z/9474656-32-6.jpg?t=%E7%B4%A7%E6%80%A5%E6%95%91%E5%91%BD%21%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%21%7BAn%7D%E4%B8%AD%2C%E3%80%901%E3%80%91a2%2Ba3%2Ba11%2Ba12%EF%BC%9D40%2C%E6%B1%82a7%EF%BC%9B%E3%80%902%E3%80%91a2%2Ba3%2Ba4%2Ba5%EF%BC%9D34%2Ca2%C3%97a5%EF%BC%9D52%2C%E6%B1%82%E5%85%AC%E5%B7%AEd%E5%8F%A6%E4%B8%80%E9%81%93%EF%BC%9Aa1%EF%BC%9D1%EF%BC%8F3%2Ca2%2Ba5%EF%BC%9D4%2CAn%EF%BC%9D33%2C%E6%B1%82n%E7%AC%AC3%E9%81%93%EF%BC%9AS20%EF%BC%9D120%2C%E5%88%99a2%2Ba19%EF%BC%9D%3F%E7%AC%AC4%E9%81%93%EF%BC%9Aa4%EF%BC%9D0.8%2CAn%EF%BC%9D2.2%2C%E6%B1%82a51%2Ba52...%2Ba80%EF%BC%9D%3F%E7%AC%AC5%E9%81%93)
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