To find angle $ \alpha $ use formula:

$$ \sin \left( \frac{ \alpha }{ 2 } \right) = \dfrac{ d_2 }{ d_1 } $$

After substituting $d_2 = 15\, \text{cm}$ and $d_1 = 40\, \text{cm}$ we have:

$$ \sin \left( \frac{ \alpha }{ 2 } \right) = \dfrac{ 15\, \text{cm} }{ 40\, \text{cm} } $$ $$ \sin \left( \frac{ \alpha }{ 2 } \right) = \frac{ 3 }{ 8 } $$ $$ \frac{ \alpha }{ 2 } = \arcsin\left( \frac{ 3 }{ 8 } \right) $$ $$ \frac{ \alpha }{ 2 } = 22.0243^o $$$$ \alpha = 22.0243^o \cdot 2 $$$$ \alpha = 44.0486^o $$

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