Ta có: \(2P=2+\dfrac{2}{2^0}+\dfrac{3}{2^1}+...+\dfrac{1992}{2^{1990}}\)
\(\Rightarrow2P-P=\left(2+\dfrac{2}{2^0}+\dfrac{3}{2^1}+...+\dfrac{1992}{2^{1990}}\right)-\left(\dfrac{1}{2^0}+\dfrac{2}{2^1}+...+\dfrac{1992}{2^{1991}}\right)\)
\(=2-\dfrac{1992}{2^{1991}}+\left(\dfrac{1}{2^0}+\dfrac{1}{2^1}+...+\dfrac{1}{2^{1990}}\right)\)
\(=2-\dfrac{1992}{2^{1991}}+2-\dfrac{1}{2^{1990}}< 4\)