STEP 1: find angle $ \beta $

To find angle $ \beta $ use formula:

$$ \alpha + \beta = 90^o $$

After substituting $ \alpha = 60^o $ we have:

$$ 60^o + \beta = 90^o $$ $$ \beta = 90^o - 60^o $$ $$ \beta = 30^o $$

STEP 2: find height $ h $

To find height $ h $ use formula:

$$ h = 2 \cdot r $$

After substituting $r = 21\, \text{cm}$ we have:

$$ h = 2 \cdot 21\, \text{cm} $$ $$ h = 42\, \text{cm} $$

STEP 3: find diagonal $ d2 $

To find diagonal $ d2 $ use formula:

$$ \sin \left( \frac{ \beta }{ 2 } \right) = \dfrac{ h }{ d_2 } $$

After substituting $\beta = 30^o$ and $h = 42\, \text{cm}$ we have:

$$ \sin \left( \frac{ 30^o }{ 2 } \right) = \dfrac{ h }{ d_2 } $$ $$ \sin( 15^o ) = \dfrac{ 42\, \text{cm} }{ d_2 } $$ $$ 0.2588 = \dfrac{ 42\, \text{cm} }{ d_2 } $$ $$ d_2 = \dfrac{ 42\, \text{cm} }{ 0.2588 } $$ $$ d_2 = 162.2755\, \text{cm} $$

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