DE || AB=> CE/EB=CD/AD
DF||AB => AF/FB=AD/DC Adunam relatiile si obtinem:
CE/EB+AF/FB=CD/AD+AD/DC
CE/EB+AF/FB=CD+AD/AD+DC
CE/EB+AF/FB=AC/AC=1
CE/EB+AF/FB=1
Aici adunam numaratorul la numitor si obtinem:CE/EB+CE + AF/FB+AF=1
EB+CE=BC si FB+AF=AB.
DECI,CE/BC+AF/AB=1