Ta có : $\widehat{B}$ + $\widehat{A}$ + $\widehat{C}$ = `180^o` ( tổng 3 góc Δ )
`70^o ` + $\widehat{A}$ + $\widehat{C}$ = `180^o`
$\widehat{A}$ + $\widehat{C}$ = `180^o - 70^o = 110^o`
`2` . $\widehat{A_{2}}$ + `2` . $\widehat{C_{1}}$ = `110^o`
`2` ( $\widehat{A_{2}}$ + $\widehat{C_{1}}$ ) = `110^o`
$\widehat{A_{2}}$ + 2$\widehat{C_{1}}$ = `110^o : 2 = 55^o`
Ta có : $\widehat{A_{2}}$ + 2$\widehat{C_{1}}$ + $\widehat{AIC}$ = `180^o` ( tổng 3 góc Δ )
`55^o` + $\widehat{AIC}$ = `180^o`
=> $\widehat{AIC}$ =` 180^o - 55^o = 125^o`
