STEP 1: find side $ a $
To find side $ a $ use formula:
$$ d = 2 \cdot a $$After substituting $ d = \frac{ 5 }{ 2 } $ we have:
$$ \frac{ 5 }{ 2 } = 2 \cdot a $$ $$ a = \dfrac{ \frac{ 5 }{ 2 } }{ 2 } $$ $$ a = \frac{ 5 }{ 4 } $$STEP 2: find incircle radius $ r $
To find incircle radius $ r $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ a = \frac{ 5 }{ 4 } $ we have:
$$ r = \dfrac{ \sqrt{ 3 } \cdot \frac{ 5 }{ 4 } }{ 2 } $$$$ r = \dfrac{ \frac{ 5 \sqrt{ 3}}{ 4 } }{ 2 } $$ $$ r = \frac{ 5 \sqrt{ 3}}{ 8 } $$