<s>[INST] The circumference of the circle is $C$, what is the area of circle in terms of $C$? The circumference of the circle is $C$, what is the area of circle in terms of $C$?
a). $\dfrac {C^2}{4\pi }$
b). $2\pi C$
c). $\dfrac {4}{3} \pi C^2$
d). $2\pi C^2$
My Attempt:
$$\textrm {Circumference}=2\pi r$$
$$C=2\pi r$$
$$\dfrac {C}{2\pi }= r$$
Now,
 Area$=\pi r^2$
$$=\pi \dfrac {C^2}{4\pi^2}$$
 [/INST] Your attempt is correct, you should just cancel out the $\pi$ in the expression:
$$\require{cancel} \dfrac {\pi C^2}{4\pi^2} = \dfrac {\bcancel{\pi} C^2}{4 \bcancel{\pi} \pi} = \dfrac {C^2}{4\pi}$$
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