B1:tính nhanh D=1/5.8+1/8.11+1/11.14+…+1/605.608 E=1/30+1/42+1/56+1/72+1/90+1/100

2 bình luận về “B1:tính nhanh D=1/5.8+1/8.11+1/11.14+…+1/605.608 E=1/30+1/42+1/56+1/72+1/90+1/100”

1. D=1/(5.8)+1/(8.11)+1/(11.14)+…+1/(605.608)

   =>D×3=3/(5.8)+3/(8.11)+3/(11.14)+…+3/(605.608)

   =>D×3=1/5-1/8+1/8-1/11+1/11-1/14+…+1/605-1/608

   =>D×3=1/5-1/608

   =>D×3=603/3040

   =>D=603/(3040÷3)

   =>D=201/3040

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   E = 1/(30) + 1/(42) + 1/(56) + 1/(72) + 1/(90)+  1/(100)

   => E = 1/(5.6) + 1/(6.7) + 1/(7.8) + 1/(8.9) + 1/(9.10) + 1/(100)

   => E = 1/5 – 1/6 + 1/6 – 1/7 + 1/7 – 1/8 + 1/8 – 1/9 + 1/9 – 1/(10) + 1/(100)

   => E = 1/5 – 1/(10) + 1/(100)

   => E=  1/(10) + 1/(100)

   => E = (11)/(100)

   $\text{@ngquynhchi331}$

   Trả lời
2. Ta có: D = 1/(5.8) + 1/(8.11) + 1/(11.14) + … + 1/(605.608)

   => D = 1/3 (1/5 – 1/8 + 1/8 – 1/(11) + 1/(11) – 1/(14) + … + 1/(605) – 1/(608))

   => D = 1/3 (1/5 – 1/(608))

   => D=  1/3 . (603)/(3040)

   => D = (201)/(3040)

   Vậy D = (201)/(3040)

   ———————-

   Ta có: E = 1/(30) + 1/(42) + 1/(56) + 1/(72) + 1/(90)+  1/(100)

   => E = 1/(5.6) + 1/(6.7) + 1/(7.8) + 1/(8.9) + 1/(9.10) + 1/(100)

   => E = 1/5 – 1/6 + 1/6 – 1/7 + 1/7 – 1/8 + 1/8 – 1/9 + 1/9 – 1/(10) + 1/(100)

   => E = 1/5 – 1/(10) + 1/(100)

   => E=  1/(10) + 1/(100)

   => E = (11)/(100)

   Vậy E = (11)/(100)

   $#duong612009$

   Trả lời