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[tex]k = 2\pi.r \\ k = 2. \frac{22}{7} .21 \\ k = 2 \times 22 \times 3 \\ k = 2 \times 66 \\ k = 132 \: cm[/tex]

keliling = 22/7 x diameter
keliling = 22/7 x ( 21cm + 21cm) = 22/7 x 42 cm = c. 132 cm