[tex]\it lg2=a\ \ \ \ \ (1)\\ \\ log_27=b \Rightarrow \dfrac{lg7}{lg2}=b\ \stackrel{(1)}{\Longrightarrow}\ \dfrac{lg7}{a}=b \Rightarrow lg7=ab\ \ \ \ \ (2)\\ \\ \\ lg56=lg8\cdot7=lg8+lg7=lg2^3+lg7=3lg2+lg7\ \stackrel{(1),(2)}{=}\ 3a+ab[/tex]
