Question

tính B=1/120-2/30*33-2/33*36-...-2/117*120

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Answer 1

\(B-\left(\frac{1}{30.33}+\frac{1}{33.36}+...+\frac{1}{117.120}\right)=\frac{1}{120}-\frac{3}{30.33}-\frac{3}{33.36}-...-\frac{3}{117.120}\)

\(B-\frac{1}{3}\left(\frac{1}{30}-\frac{1}{33}+\frac{1}{33}-...+\frac{1}{117}-\frac{1}{120}\right)=\frac{1}{120}-\left(\frac{1}{30}-\frac{1}{33}+\frac{1}{33}-...-\frac{1}{120}\right)\)

\(\Rightarrow B-\frac{1}{3}\left(\frac{1}{30}-\frac{1}{120}\right)=\frac{1}{120}-\frac{1}{30}+\frac{1}{120}\)

\(\Rightarrow B=\frac{1}{60}-\frac{1}{30}+\frac{1}{3}\left(\frac{1}{30}-\frac{1}{120}\right)=-\frac{1}{120}\)

Answer 2

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