P=2s+b
s-side
b=base
42=2(10)+b
42=20+b
b=22 units
There is a theorem in Math, which is that the side is the same as the radius.
radius=10 units
Diameter is twice the radius so:
diameter=20 units.
The diameter represents the altitude of the triangle
A=1/2bh
A=1/2(22)(10)
A=1/2(220)
A=110 units²
Hope this helps =)
