`\hat{BAK}=1/ 2 (sđ\stackrel\frown{BK}-sđ\stackrel\frown{BI})`
`\hat{CAK}=1/ 2 (sđ\stackrel\frown{DK}-sđ\stackrel\frown{CI})`
Mà `\hat{BAK}=\hat{CAK}` (vế trái = vế trái)
`=>1/ 2 (sđ\stackrel\frown{BK}-sđ\stackrel\frown{BI})=1/ 2 (sđ\stackrel\frown{DK}-sđ\stackrel\frown{CI})` (vế phải = vế phải)
`=>1/ 2 sđ\stackrel\frown{BK}-1/ 2sđ\stackrel\frown{BI}=1/ 2 sđ\stackrel\frown{DK}-sđ\stackrel\frown{CI}` (nhân `1/ 2` vào bỏ ngoặc)
`=>1/ 2 sđ\stackrel\frown{BK}+1/ 2sđ\stackrel\frown{CI}=1/ 2 sđ\stackrel\frown{BK}+1/ 2sđ\stackrel\frown{BI}` (chuyển vế đổi dấu)
`=>1/ 2 (sđ\stackrel\frown{BK}+sđ\stackrel\frown{CI}=1/ 2 (sđ\stackrel\frown{DK}+sđ\stackrel\frown{BI})`
