$\widehat{A}-\widehat{B}={{18}^{0}}\Rightarrow \widehat{A}={{18}^{0}}+\widehat{B}$

$\widehat{B}-\widehat{C}={{18}^{0}}\Rightarrow \widehat{C}=\widehat{B}-{{18}^{0}}$

Mặt khác tổng ba góc trong một tam giác bằng 180$^{0}$ ta có:

$\widehat{A}+\widehat{B}+\widehat{C}={{180}^{0}}$

${{18}^{0}}+\widehat{B}+\widehat{B}+\widehat{B}-{{18}^{0}}={{180}^{0}}$

$3\widehat{B}={{180}^{0}}$

$\Rightarrow \widehat{B}={{60}^{0}}\Rightarrow \widehat{C}=$42$^{0}$