a = 2* R* sin( )$$a$$ = $$2\cdot R\cdot sin(\frac{180}{n})$$ 
a = 8* R* cos( )* cos( )* sin( )$$a$$ = $$8\cdot R\cdot cos(\frac{45}{n})\cdot cos(\frac{90}{n})\cdot sin(\frac{45}{n})$$ a = 8* 3* cos( )* cos( )* sin( )$$a$$ = $$8\cdot 3\cdot cos(\frac{45}{6})\cdot cos(\frac{90}{6})\cdot sin(\frac{45}{6})$$ a = 8* 3* cos( )* cos( )* sin( )$$a$$ = $$8\cdot 3\cdot cos(\frac{15}{2})\cdot cos(\frac{90}{6})\cdot sin(\frac{45}{6})$$ a = 8* 3* 0.99144* cos( )* sin( )$$a$$ = $$8\cdot 3\cdot 0.99144\cdot cos(\frac{90}{6})\cdot sin(\frac{45}{6})$$ a = 8* 3* 0.99144* cos(15)* sin( )$$a$$ = $$8\cdot 3\cdot 0.99144\cdot cos(15)\cdot sin(\frac{45}{6})$$ a = 8* 3* 0.99144* 0.96593* sin( )$$a$$ = $$8\cdot 3\cdot 0.99144\cdot 0.96593\cdot sin(\frac{45}{6})$$ a = 8* 3* 0.99144* 0.96593* sin( )$$a$$ = $$8\cdot 3\cdot 0.99144\cdot 0.96593\cdot sin(\frac{15}{2})$$ a = 8* 3* 0.99144* 0.96593* 0.13053$$a$$ = $$8\cdot 3\cdot 0.99144\cdot 0.96593\cdot 0.13053$$
a = 3.0000857703546$$a$$ = $$3.0000857703546$$
a = 3.00009$$a$$ = $$3.00009$$