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\(\frac{1}{2}+\frac{5}{6}+\frac{11}{12}+\frac{19}{20}+...+\frac{89}{90}\)
\(=1-\frac{1}{2}+1-\frac{1}{6}+1-\frac{1}{12}+1-\frac{1}{20}+...+1-\frac{1}{90}\)
\(=9-\left(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\right)\)
\(=9-\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\right)\)
\(=9-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{9}-\frac{1}{10}\right)\)
\(=9-\left(1-\frac{1}{10}\right)\)
\(=9-\frac{9}{10}=\frac{81}{10}\)
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Vì E nằm trong đoạn thẳng CD=> E nằm giữa C và D
Ta có CE+ED=CD
Thay số 5+ED= 8
ED=8-3
ED=5 cm
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\(15:x-2=3\)
\(15:x=3+2\)
\(15:x=5\)
\(x=15:5\)
\(x=3\)
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29-(10+29)=x-(27-9)
20-10-9=x-27+9
10-9=x-36
1=x-36
x-36=1
x=1+36
x=37
=21+2+3+...+49+50
=2(1+50)*50/2
=21275
21 x 22 x 23 x 24 x...x 250 = 21x2x3x4x...x50