2/1X3+2/3X5+2/5x7+.+2/19x21全过程(最好带原因),
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![2/1X3+2/3X5+2/5x7+.+2/19x21全过程(最好带原因),](/uploads/image/z/11482421-5-1.jpg?t=2%2F1X3%2B2%2F3X5%2B2%2F5x7%2B.%2B2%2F19x21%E5%85%A8%E8%BF%87%E7%A8%8B%EF%BC%88%E6%9C%80%E5%A5%BD%E5%B8%A6%E5%8E%9F%E5%9B%A0%EF%BC%89%2C)
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2/1X3+2/3X5+2/5x7+.+2/19x21全过程(最好带原因),
2/1X3+2/3X5+2/5x7+.+2/19x21全过程(最好带原因),
2/1X3+2/3X5+2/5x7+.+2/19x21全过程(最好带原因),
=2*(1/1*3+1/3*5+.)=2*1/2*(1-1/3+1/3-1/5.+1/19-1/21)=20/21