Từ \(\dfrac{1}{a}-\dfrac{1}{b}=1\Leftrightarrow\dfrac{b-a}{ab}=1\Leftrightarrow b-a=ab\)
Ta có:
\(P=\dfrac{a-2ab-b}{2a+3ab-2b}=\dfrac{a-2\left(b-a\right)-b}{2a+3\left(b-a\right)-2b}\)
\(P=\dfrac{a-2b+2a-b}{2a+3b-3a-2b}=\dfrac{3a-3b}{b-a}=\dfrac{3\left(a-b\right)}{-\left(a-b\right)}=-3\)