a. ta co goc ABH= goc BHD ( tam giac AHB= tam giac DBH)
ma goc ABH va goc BHD nam o vi tri so le trong
nen AB//HD (đcpcm)

c. xet tam giac BAO va tam giac HDO ta co
AB=DH ( tam giac ABH= tam giac DBH)
goc OBA= goc OHD (2 goc so le trong va AB//HD)
goc OAB= goc ODH ( 2 goc so le trong va AB//HD)
--> 2 tam giac = nhau ( g=c=g) --> BO= OH ( 2 canh tuong ung )
--> O la trung diem BH ( O thuoc BH)
ta co : goc BDH= goc BAH ( tam giac BDH= tam giac AHB )
ma goc BDH = 35 °( gt)
nen goc BAH=35 °
ta co: goc BAH+ goc HAC=90 ( 2 goc ke phu) goc HAC+goc ACB=90 ( tam giac AHC vuong tai H )
--> goc BAH= goc ACB
--> goc ACB=45°