请教考研数学排列组合题1从1,2,3,4,.20这20个自然数中任取3个不同的数,使他们成等差数列,这样的等差数列共有( )A 90个 B 120个 C 200个 D 180个 E 190个2、有甲乙丙三项任务,甲需要2人承担,
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![请教考研数学排列组合题1从1,2,3,4,.20这20个自然数中任取3个不同的数,使他们成等差数列,这样的等差数列共有( )A 90个 B 120个 C 200个 D 180个 E 190个2、有甲乙丙三项任务,甲需要2人承担,](/uploads/image/z/5390870-14-0.jpg?t=%E8%AF%B7%E6%95%99%E8%80%83%E7%A0%94%E6%95%B0%E5%AD%A6%E6%8E%92%E5%88%97%E7%BB%84%E5%90%88%E9%A2%981%E4%BB%8E1%2C2%2C3%2C4%2C.20%E8%BF%9920%E4%B8%AA%E8%87%AA%E7%84%B6%E6%95%B0%E4%B8%AD%E4%BB%BB%E5%8F%963%E4%B8%AA%E4%B8%8D%E5%90%8C%E7%9A%84%E6%95%B0%2C%E4%BD%BF%E4%BB%96%E4%BB%AC%E6%88%90%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%2C%E8%BF%99%E6%A0%B7%E7%9A%84%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%E5%85%B1%E6%9C%89%EF%BC%88++++%EF%BC%89A+90%E4%B8%AA++B+120%E4%B8%AA++C+200%E4%B8%AA++D+180%E4%B8%AA+++E+190%E4%B8%AA2%E3%80%81%E6%9C%89%E7%94%B2%E4%B9%99%E4%B8%99%E4%B8%89%E9%A1%B9%E4%BB%BB%E5%8A%A1%2C%E7%94%B2%E9%9C%80%E8%A6%812%E4%BA%BA%E6%89%BF%E6%8B%85%2C)
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