1/ Ta có: ^B+^C=1800-^A=1200 => 1/2(^B+^C)=600 => ^IBC+^ICB=60 => ^BIC=1200
=> ^DIB=1800-^BIC=600
2/ IK là phân giác ^BIC => ^I1=^I2=600.
^I4=1800-^BIC=600 => ^I4=^I1=600
=> \(\Delta\)CKI=\(\Delta\)CEI (g.c.g) => IK=IE (2 cạnh tương ứng)
3/ Ta có: \(\Delta\)BKI=\(\Delta\)BDI (g.c.g) => IK=ID (2 cạnh tương ứng)
Lại có: IK=IE (cmt) => IE=ID.