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a) \(\left(\frac{4}{3}-\frac{4}{6}\right)+\left(\frac{4}{6}-\frac{4}{9}\right)+\left(\frac{4}{9}-\frac{4}{10}\right)+\left(\frac{4}{12}-\frac{4}{15}\right)\)
\(=\frac{4}{15}-\frac{4}{3}=\frac{-16}{15}\)
C) bạn chỉ ần bỏ các số giống nhau thôi nhé
= 1
b)
1/2 x y + 1/3 x y + 1/4 x y = 11/4
y x ( 1/2 + 1/3 + 1/4 ) = 11/4
y x 13/12 = 11/4
y = 11/4 : 13/12
y = 11/4 x 12/13
y = 33/13
^^ Học tốt!
\(\frac{1}{2}\times Y+\frac{1}{3}\times Y+\frac{1}{4}\times Y=\frac{11}{4}\)
\(3Y=\frac{11}{4}\div\left(\frac{1}{2}+\frac{1}{3}+\frac{1}{4}\right)\)
\(3Y=\frac{33}{13}\)
\(Y=\frac{33}{13}\div3\)
\(Y=\frac{11}{13}\)
~ Chúc bạn học tốt
\(=\frac{4}{2x4}+\frac{4}{4x6}+\frac{4}{6x8}+...+\frac{4}{18x20}\)
\(=2\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+...+\frac{1}{18}-\frac{1}{20}\right)\)
\(=2x\left(\frac{1}{2}-\frac{1}{20}\right)\\ =2x\frac{9}{20}\\ =\frac{9}{10}\)
Đặt \(A=\frac{4}{2.4}+\frac{4}{4.6}+\frac{4}{6.8}+...+\frac{4}{16.18}\)
\(A=\frac{4-2}{2.4}+\frac{6-4}{4.6}+\frac{8-6}{6.8}+....+\frac{18-16}{16.18}\)
\(A=\frac{4}{2}.\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{16}-\frac{1}{18}\right)\)
\(A=\frac{4}{2}.\left(\frac{1}{2}-\frac{1}{18}\right)\)
\(A=\frac{4}{2}.\frac{4}{9}\)
\(\Rightarrow A=\frac{8}{9}\)
\(\frac{4}{2.4}+\frac{4}{4.6}+\frac{4}{6.8}+...+\frac{4}{16.18}\)
\(=\frac{4}{2}\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{16}-\frac{1}{18}\right)\)
\(=2\left(\frac{1}{2}-\frac{1}{18}\right)\)
\(=2.\frac{4}{9}\)
\(=\frac{8}{9}\)