\(\dfrac{9\left(9-1\right)}{2}=\dfrac{72}{2}=36\left(góc\right)\)

\(C.36\)

\(1h20'\times20=\dfrac{80}{3}h\)

\(=\dfrac{5}{9}\left(\dfrac{2}{7}+\dfrac{7}{7}\right)-\dfrac{8}{3}\)

\(=\dfrac{5}{9}-\dfrac{8}{3}=-\dfrac{19}{9}\)

\(3s=3-3^2+3^3-3^4+...+3^{100}\)

\(4s=\left(3-3^2+3^3-3^4+...+3^{101}\right)+\left(1-3+3^2-3^3+...+3^{100}\right)\)

\(4s=1\)

\(s=\dfrac{1}{4}>\dfrac{1}{5}\)

\(=-\dfrac{3}{7}\left(\dfrac{5}{11}+\dfrac{6}{11}-3\right)\)

\(=-\dfrac{3}{7}\times\left(-2\right)=\dfrac{6}{7}\)

Diện tích là:

\(\dfrac{2}{3}\times\dfrac{3}{4}=\dfrac{1}{2}m^2\)

\(a=\left|x-2021\right|+\left|x-2022\right|\)

\(=\left|x-2021\right|+\left|2022-x\right|\)

\(\ge\left|x-2021+2022-x\right|=1\)

\(A=1\Leftrightarrow\left(x-2021\right)\left(2022-x\right)\ge0\)

\(\Rightarrow2021\le x\le2022\)