notam ABC triunghiul
Teorema inaltimii
BD²=AD·DC ⇒DC=BD²/AD
Teorema catetei
AB²=AD·AC
BC²=DC·AC
AB/BC=3/7
BD=42
AB²/BC²=AD·AC/DC·AC=AD/DC=3²/7²=9/49⇒ AD/DC=9/49
dar DC=BD²/AD
AD/(BD²/AD)=9/49
AD²/BD²=9/49
AD²/42²=9·49
AD=√(9x42²/49)=3x42/7=18
AD/DC=9/49
18/DC=9/49
DC=98