a) tam giac  abc vuong can tai c
=> goc b = goc c= 45 do
pm // bc => goc mpa= goc c= 90 (dong vi)
xet tam giac mpa co goc mpa =90 do
=> goc a + goc pma = 90 do
=> goc pma = 90 do - goc a= 90 do - 45 do= 45 do
=> goc pma = goc a =45 do
=> tam giac apm vuong can tai p
=> mp=pa=cq 
tu giac mqcp cos mp // bc hay mp // qc
qc= mp => tu giac mqcp la hinn binh hanh ( dhnb)
=> qm// cp ( t/c)
=> goc bqm = goc c = 90 do
tam giac bqm co goc bqm = 90 do
=> goc b+ bmq = 90 do
=> bmq= 45 do= bmp
=> tam giac bqm vuong can tai q
b) tu giac mqcp la hinh binh hanh (cmt) co goc c = 90 do
=> tu giac mqc la HCN ( dhnt)