22 - 22 + 22 = ?
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<Hình bạn tự vẽ nha :) >
a) Vì tia OC nằm giữa OA và OB nên
\(\widehat{AOB}=\widehat{AOC}+\widehat{COB}\)
\(\Rightarrow\widehat{COB}=\widehat{AOB}-\widehat{AOC}\)
\(\Leftrightarrow\widehat{COB}=80^o-54^o=26^o\)
b) OC là tia phân giác \(\widehat{AOD}\)nên\(\widehat{COA}=\widehat{COD}=54^o\)
Vậy \(\widehat{AOD}=\widehat{COA}+\widehat{COD}=54^o+54^o=108^o\)
Ta có: \(\widehat{DOB}=\widehat{COD}-\widehat{COB}=54^o-26^o=28^o\)
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\(\dfrac{22}{5\times9}+\dfrac{22}{9\times13}+\dfrac{22}{13\times17}+\dfrac{22}{17\times21}+\dfrac{22}{21\times25}\\ =\dfrac{2}{11}\times\left(\dfrac{4}{5\times9}+\dfrac{4}{9\times13}+\dfrac{4}{13\times17}+\dfrac{4}{17\times21}+\dfrac{4}{21\times25}\right)\\ =\dfrac{2}{11}\times\left(\dfrac{1}{5}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{17}+\dfrac{1}{17}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{25}\right)\\ =\dfrac{2}{11}\times\left(\dfrac{1}{5}-\dfrac{1}{25}\right)\\ =\dfrac{2}{11}\times\dfrac{5-1}{25}\\ =\dfrac{2}{11}\times\dfrac{4}{25}=\dfrac{8}{275}\)
ket qua bang 22
h minh nha ban
giúp nha