Có : \(\left\{{}\begin{matrix}\widehat{A}+\widehat{HIA}=90^0\\x+\widehat{BIK}=90^0\end{matrix}\right.\)(2 góc phụ nhau)
Mà \(\widehat{HIA}=\widehat{BIK}\)(đối đỉnh)
`=>` \(\widehat{A}=x=40^0\)
Xét `ΔHAI` và `ΔKBI`có :
\(\widehat{H}=\widehat{K}=90^0\)
`HA=KB`(gt)
\(\widehat{A}=x=40^0\)
`=> ΔHAI = ΔKBI`(g-c-g)