Theo đề bài, ta có:
A = \(\dfrac{6}{15.18}+\dfrac{6}{18.21}+\dfrac{6}{21.24}+...+\dfrac{6}{87.90}\)
A = \(2.\left(\dfrac{3}{15.18}+\dfrac{3}{18.21}+\dfrac{3}{21.24}+...+\dfrac{3}{87.90}\right)\)
A =\(2.\left(\dfrac{1}{15}-\dfrac{1}{18}+\dfrac{1}{18}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{24}+...+\dfrac{1}{87}-\dfrac{1}{90}\right)\)
A = \(2.\left(\dfrac{1}{15}-\dfrac{1}{90}\right)\)= \(\dfrac{1}{8}\)
A= \(\dfrac{6}{15.18}+\dfrac{6}{18.21}+...+\dfrac{6}{87.90}\)
A= \(2\left(\dfrac{3}{15.18}+\dfrac{3}{18.21}+...+\dfrac{3}{87.90}\right)\)
A= \(2\left(\dfrac{1}{15}-\dfrac{1}{18}+\dfrac{1}{18}-\dfrac{1}{21}+...+\dfrac{1}{87}-\dfrac{1}{90}\right)\)
A= \(2\left(\dfrac{1}{15}-\dfrac{1}{90}\right)\)
A= 2. \(\dfrac{1}{16}\)
A= \(\dfrac{1}{8}\)
A =\(\dfrac{6}{15.18}+\dfrac{6}{18.21}+\dfrac{6}{21.24}+...+\dfrac{6}{87.90}\)
= \(\dfrac{6}{3}.\left(\dfrac{3}{15.18}+\dfrac{3}{18.21}+\dfrac{3}{21.14}+...+\dfrac{3}{87.90}\right)\)
=\(\dfrac{6}{3}.\left(\dfrac{1}{15}-\dfrac{1}{18}+\dfrac{1}{18}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{24}+...+\dfrac{1}{87}+\dfrac{1}{90}\right)\)
=\(\dfrac{6}{3}.\left(\dfrac{1}{15}-\dfrac{1}{90}\right)\)
= \(\dfrac{6}{3}.\dfrac{1}{18}=\dfrac{1}{9}\)
A=\(\dfrac{6}{15.18}\)+\(\dfrac{6}{18.21}\)+\(\dfrac{6}{21.24}\)+....+\(\dfrac{6}{87.90}\)
A=3.(\(\dfrac{3}{15.18}\)+\(\dfrac{3}{18.21}\)+\(\dfrac{3}{21.24}\)+....+\(\dfrac{3}{87.90}\))
A=3.(\(\dfrac{1}{15.18}\)+\(\dfrac{1}{18.21}\)+\(\dfrac{1}{21.24}\)+....+\(\dfrac{1}{87.90}\))
A=3.(\(\dfrac{1}{15}\)-\(\dfrac{1}{18}\)+\(\dfrac{1}{18}\)-\(\dfrac{1}{21}\)+\(\dfrac{1}{21}\)-\(\dfrac{1}{24}\)+....+\(\dfrac{1}{87}\)-\(\dfrac{1}{90}\))
A=3.(\(\dfrac{1}{15}\)-\(\dfrac{1}{90}\))
A=3.\(\dfrac{1}{18}\)
=>A=\(\dfrac{1}{6}\)
\(\dfrac{1}{18}\)minh rut gon tu \(\dfrac{5}{90}\) ; \(\dfrac{1}{6}\)minh rut gon tu \(\dfrac{3}{18}\)
CHUC BAN LAM BAI TOT
6/15.18+6/18.21+6/21.24+...+6/87.90
=2(1/15.18+1/18.21+1/21.24+...+1/87.90)
=2(1/15-1/18+1/18-1/21+1/21-1/24+...+1/87-1/90)
=2(1/15-1/90)
=2.1/18
=1/9
lấy tử là 1 ở đâu vậy mấy bạn
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