Question

determinati lungimea cercului si aria discului cu diametru egal cu:
a 14cm b10m c9 m ​

Answer 1

 

$\displaystyle\bf\\Metoda~1:\\a)\\L_{cerc} =\pi D=14\pi~cm\\\\A_{disc}=\frac{\pi D^2}{4}=\frac{\pi\times 14^2}{4}=\frac{196\pi}{4}=49\pi~cm^2\\\\b)\\L_{cerc} =\pi D=10\pi~m\\\\A_{disc}=\frac{\pi D^2}{4}=\frac{\pi\times10^2}{4}=\frac{100\pi}{4}=25\pi~m^2\\\\c)\\L_{cerc} =\pi D=9\pi~m\\\\A_{disc}=\frac{\pi D^2}{4}=\frac{\pi\times9^2}{4}=\frac{81\pi}{4}=20,25\pi~m^2$

.

$\displaystyle\bf\\Metoda~2:\\a)\\D=14~cm\\\\R=\frac{D}{2}=\frac{14}{2}=7~cm\\\\L_{cerc}=2\pi R=2\pi\times7=14\pi~cm\\\\A{disc}=\pi R^2=\pi\times 7^2=49\pi~cm^2\\\\b)\\D=10~m\\\\R=\frac{D}{2}=\frac{10}{2}=5~m\\\\L_{cerc}=2\pi R=2\pi\times5=10\pi~m\\\\A{disc}=\pi R^2=\pi\times 5^2=25\pi~m^2\\\\c)\\D=9~m\\\\R=\frac{D}{2}=\frac{9}{2}=4,5~m\\\\L_{cerc}=2\pi R=2\pi\times4,5=9\pi~m\\\\A{disc}=\pi R^2=\pi\times 4,5^2=20,25\pi~m^2$