h^2=a^2-(\frac{1}{2}a)^2
h^2=12^2-(\frac{1}{2}*12)^2
h^2=144-6^2=144-36=108
h=\sqrt{108}=\sqrt{36*3}=6\sqrt{3}
P=\frac{1}{2}ah=\frac{1}{2}*12*6\sqrt{3}=36\sqrt{3}cm^2
…
r_1=18:2=9
r_2=10:2=5
\Pi r_1^2-\Pi r_2^2=\Pi(r_1^2-r_2^2)=(9^2-5^2)\Pi=(81-25)\Pi=(81-25)\Pi=56\Pi cm^2