Question

1/15.18+1/18.21+1/21.24+..+1/87.90

tính nhanh

 

Answer 1

Ta có : \(\frac{1}{15.18}+\frac{1}{18.21}+\frac{1}{21.24}+...+\frac{1}{87.90}\)

= \(\frac{1}{3}\left(\frac{3}{15.18}+\frac{3}{18.21}+\frac{3}{21.24}+...+\frac{3}{87.90}\right)\)

= \(\frac{1}{3}\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+...+\frac{1}{87}-\frac{1}{90}\right)\)

= \(\frac{1}{3}\left(\frac{1}{15}-\frac{1}{90}\right)\)

= \(\frac{1}{3}.\frac{1}{18}\)

= \(\frac{1}{54}\)

Answer 2

\(\frac{1}{15.18}+\frac{1}{18.21}+....+\frac{1}{87.90}\)

\(=\frac{1}{3}.\left(\frac{1}{15}-\frac{1}{90}\right)=\frac{1}{54}\)

Answer 3

= 1-1/15+(1/18-1/18) + (1/21-1/21)+...+(1/87-1/87) +1/90

= 1-(1/15-1/90)= 1-1/18 = 17/18

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Answer 4

chết quên mk sai rùi

Answer 5

= 1-1/15+(1/18-1/18) + (1/21-1/21)+...+(1/87-1/87) +1/90

= 1-(1/15-1/90)= 1-1/18 = 17/18

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