Fie ΔABC;
AD _|_ BC; D ∈BC;
AD-h
DM _|_AB; M ∈ AB ---->[DM]=[DN] (1)
DN _|_AC; N ∈ AC
Fie ΔADB-dreptunghic (m<ADB=90)
DM _|_AB => m<BMD=m<DMA=90
Fie ΔADC-dreptunghic (m<ADC=90)
DN_|_AC => m<DNC=m<DNA=90
ΔBDM
ΔDNC ---> {m<BMD=m<DNC=90
{Din (1)
C.U ===> ΔBDM = ΔDNC
(<MBD=<DCN) (2)
ΔABC
Din (2) ; (<B=<C) ===> ΔABC-isoscel
[AB]=[AC] c.c.t.d.