Răspuns:
1.
BD/DC=AB/AC (t.bisect.)
BD+DC=8
BD/DC=6/10=3/5 =>BD=3DC/5
3DC/5 + DC = 8
3DC+5DC=40
8DC=40 =>DC=5 cm =>BD=3 cm
2.
BD/DC=AB/AC (idem)
BD/DC=20/30=2/3 =>BD=2DC/3
2DC/3 + DC = 25
2DC+3DC=75
5DC=75 =>DC=15 cm=>BD=10 cm
tr.CED~tr.CAB =>CD/CB=ED/AB (t.Thales)
15/25=ED/20 =>ED=300/25=12 cm
CE/CA=CD/CB
CE/30=15/25 =>CE=18 cm
P.tr.DEC=18+12+15=45 cm
3.
AD/DC=AB/BC
7/6=21/BC =>BC=18 cm