c) Có IH//AB \Rightarrow \frac{AB}{IH}=\frac{CB}{CI} (1)
Có IH//CG \Rightarrow \frac{CB}{CI}=\frac{BG}{HG}=\frac{HB+HG}{HG}=1+\frac{HB}{HG} (2)
Có \triangle ABH \sim \triangle CGH \Rightarrow \frac{HB}{HG}=\frac{AB}{CG} (3)
(1)(2)(3) \Rightarrow \frac{AB}{IH}=1+\frac{AB}{CG}
\Leftrightarrow \frac{AB}{IH}=\frac{AB}{AB}+\frac{AB}{CG}
\Leftrightarrow \frac1{IH}=\frac1{AB}+\frac1{CG}