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\(\frac{1}{7}.\frac{-7}{18}=\frac{-1}{18}\)
\(\frac{1}{7}.\frac{-7}{18}=\frac{-1}{18}\)
HT
\(99-88+\left(-99\right)\)
\(=\left[99+\left(-99\right)\right]-88\)
\(=0-88\)
\(=-88\)
~HọcTốt~
\(\dfrac{4}{3.5}+\dfrac{8}{5\cdot9}+\dfrac{12}{9\cdot15}+...+\dfrac{32}{n\left(n+6\right)}=\dfrac{16}{25}\)
\(2\left(\dfrac{1}{3}-\dfrac{1}{5}\right)+2\left(\dfrac{1}{5}-\dfrac{1}{9}\right)+2\left(\dfrac{1}{9}-\dfrac{1}{15}\right)+...+2\left(\dfrac{1}{n}-\dfrac{1}{n+16}\right)=\dfrac{16}{25}\)
\(2\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{15}+...+\dfrac{1}{n}-\dfrac{1}{n+16}\right)=\dfrac{16}{25}\)
\(2\left(\dfrac{1}{3}-\dfrac{1}{n+16}\right)=\dfrac{16}{25}\)
\(\dfrac{1}{3}-\dfrac{1}{n+16}=\dfrac{8}{25}\)
\(\dfrac{1}{n+16}=\dfrac{1}{75}\)
⇒ \(n+16=75\)
\(\Rightarrow n=59\)
\(-9+\left(-11\right)+21+\left(-1\right).\)
\(=\left[\left(-9\right)+\left(-11\right)+\left(-1\right)\right]+21\)
\(=-21+21\)
\(=0\)
