[tex]\bf \textit{area of a circle}\\\\
A=\pi r^2\qquad r=radius\\\\
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3)\\\\
\textit{since the diameter is 20, then the radius is half that}\\\\
A=\pi 10^2\implies A=100\pi
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4)\\\\
r=14\qquad \qquad A=14^2\pi \implies A=196\pi
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5)\\\\
\textit{since the diameter is 8, then the radius is half that}\\\\
A=\pi 4^2\implies A=16\pi \\\\
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\textit{circumference of a circle}\\\\
C=2\pi r\\\\[/tex]
[tex]\bf -------------------------------\\\\
\textit{diameter is 5, so radius is 2.5 then}\\\\
\stackrel{\textit{circumference of the circle}}{2\pi 2.5}\implies 5\pi \qquad \qquad \stackrel{\textit{circumference of the semi-circle}}{\cfrac{5\pi }{2}}
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6)\\\\
\textit{diameter is 21, so radius is 10.5 then}\\\\
\stackrel{\textit{circumference of the circle}}{2\pi 10.5}\implies 21\pi \qquad \qquad \stackrel{\textit{circumference of the semi-circle}}{\cfrac{21\pi }{2}}[/tex]
