Câu 18:
\(\int\limits^1_0[f\left(x\right)-2g\left(x\right)]dx=\int\limits^1_0f\left(x\right)dx-2\int\limits^1_0g\left(x\right)dx=3-2.\left(-4\right)=11\Rightarrow D.11\)
19/
\(\overline{z}=-3+i\Rightarrow B\)
20/
\(z_1-z_2=\left(2-3\right)+\left(-1-3\right)i=-1-4i\Rightarrow A.-4\)
21/ \(\left|z\right|=\sqrt{5^2+\left(-4\right)^2}=\sqrt{41}\Rightarrow A.41\)