The correct option is D 3CD2
Given, equilateral triangle ABC, AD⊥BC
Since, it is an equilaateral triangle, the perpendicualr also bisects the side.
Hence, BD=DC or BD=12BC=12AB
Now, In △ABD, we have
AB2=AD2+BD2
⇒AB2−BD2=AD2
⇒BC2−BD2=AD2
⇒(2CD)2−CD2=AD2
⇒3CD2=AD2