fie CF_|_AB, =>CF || AD => AFCD-dreptunghi =>CF=40rad.3m
FB=AB-CD=100m-60m=40m
In Triunghi CFB (<F=90 grd.) aplic tg (<CBF)=CF/FB=40rad.3m40m=rad.3
=>mas(<CBF)=60grd., dar <CBF si <BCD suplementare ca unghiuri alaturate lat. neparalele in trapez =>m(<BCD)=180-60=120grd.