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Ta có : \(\frac{1}{2}+\frac{5}{6}+\frac{11}{12}+...+\frac{89}{90}\)
\(=1-\frac{1}{2}+1-\frac{1}{6}+1-\frac{1}{12}+...+1-\frac{1}{90}\)
\(=9-\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\right)\)
\(=9-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{9}-\frac{1}{10}\right)\)
\(=9-\left(1-\frac{1}{10}\right)\)
\(=\frac{81}{10}\)
\(\frac{2}{3}.\frac{4}{5}+\frac{1}{3}.\frac{4}{5}=\frac{4}{5}\left(\frac{2}{3}+\frac{1}{3}\right)=\frac{4}{5}.\frac{3}{3}=\frac{4}{5}.1=\frac{4}{5}\)
\(\frac{1}{2}:\frac{3}{4}+\frac{1}{6}:\frac{3}{4}=\frac{3}{4}:\left(\frac{1}{2}+\frac{1}{6}\right)=\frac{3}{4}:\frac{2}{3}=\frac{9}{8}\)
\(\frac{2}{3}.\frac{4}{5}-\frac{1}{3}.\frac{4}{5}=\frac{4}{5}\left(\frac{2}{3}-\frac{1}{3}\right)=\frac{4}{5}.\frac{1}{3}=\frac{4}{15}\)
\(\frac{1}{2}:\frac{3}{4}-\frac{1}{6}:\frac{3}{4}=\frac{3}{4}:\left(\frac{1}{2}-\frac{1}{6}\right)=\frac{3}{4}:\frac{1}{3}=\frac{9}{4}\)
\(\frac{2}{3}.\frac{4}{5}+\frac{1}{3}.\frac{4}{5}=\left(\frac{2}{3}+\frac{1}{3}\right).\frac{4}{5}=1.\frac{4}{5}=\frac{4}{5}\)
\(\frac{1}{2}:\frac{3}{4}+\frac{1}{6}:\frac{3}{4}=\frac{1}{2}.\frac{4}{3}+\frac{1}{6}.\frac{4}{3}=\left(\frac{1}{2}+\frac{1}{6}\right).\frac{4}{3}=\frac{2}{3}.\frac{4}{3}=\frac{8}{9}\)
c,d tương tự
\(=1x2x\frac{3}{2}x\frac{4}{3}x\frac{5}{4}x\frac{6}{5}x\frac{7}{6}x\frac{8}{7}\)
\(=\frac{1x2x3x4x5x6x7x8}{1x1x2x3x4x5x6x7}=8\)
\(1:\frac{1}{2}:\frac{2}{3}:\frac{3}{4}:\frac{4}{5}:\frac{5}{6}:\frac{6}{7}:\frac{7}{8}=8\)
chúc bn học gioi!
nhae$
hihi
Lê Thị Thu Hương
h@__@
4 2/5 + 5 6/9 + 2 3/4 + 3/5 + 1/3 + 1/4
= 4 + 2/5 + 5 + 6/9 + 2 + 3/4 + 3/5 + 1/3 + 1/4
= (4 + 5 + 2) + (2/5 + 3/5) + (6/9 + 1/3) + (3/4 + 1/4)
= 11 + 1 + 1 + 1
= 11 + 1 × 3
= 11 + 3
= 14
Chú ý: hỗn số vít hơi khó, phải zô Fx nên mk ngăn = dấu cách nha
Ủng hộ mk nha ☆_☆♥_♥^_-
4 2/5 + 5 6/9 + 2 3/4 + 3/5 + 1/3 + 1/4
= 4 + 2/5 + 5 + 6/9 + 2 + 3/4 + 3/5 + 1/3 + 1/4
= (4 + 5 + 2) + (2/5 + 3/5) + (6/9 + 1/3) + (3/4 + 1/4)
= 11 + 1 + 1 + 1
= 11 + 1 × 3
= 11 + 3
= 14
các gt ll là
1
1
1
1
1
hok tốt
1/1 = 1
2/2 = 1
4/4 = 1
5/5 = 1
Học tốt
nhớ t.i.c.k