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222-9+11+12:2*14+14 = ? ( )

Toán Lớp 7: Tính nhanh tổng sau: M=$\frac{1}{2.15}$ +$\frac{1}{15.3}$ +$\frac{1}{3.21}$+…+$\frac{6}{87.90}$

Toán Lớp 7: Tính nhanh tổng sau:
M=$\frac{1}{2.15}$ +$\frac{1}{15.3}$ +$\frac{1}{3.21}$+…+$\frac{6}{87.90}$

Comments ( 2 )

  1. #Mon
    M=\frac{1}{2.15}+\frac{1}{15.3}+\frac{1}{3.21}+…+\frac{6}{87.90}
    M=\frac{6}{12.15}+\frac{6}{15.18}+…+\frac{6}{87.90}
    M=2(\frac{3}{12.15}+\frac{3}{15.18}+…+\frac{3}{87.90})
    M=2(\frac{1}{12}-\frac{1}{15}+\frac{1}{15}-\frac{1}{18}+…+\frac{1}{87}-\frac{1}{90})
    M=\2(\frac{1}{12}-\frac{1}{90})
    M=\frac{13}{90} 

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222-9+11+12:2*14+14 = ? ( )

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