Question 33486
The length of an arc, S is given by S=rA where r is the radius and A is the angle IN RADIANS:


So, we have 144 degrees. What is this in radians?


Start with one thing you NEED to learn: 
360 degree = 2 {{{pi}}} radians
--> 1 degree = {{{ (2*pi)/360 }}} radians
--> 144 degrees = {{{ 144*((2*pi)/360) }}} radians


You can simplify this out a bit if you wish here:
{{{ 144*((pi)/180) }}}
{{{ (4*pi)/5 }}}


Right then, S=rA
--> {{{ S = 12*(4*pi)/5 }}}
--> {{{ S = (48*pi)/5 }}}


So, this is equivalent to {{{ (3y-9)*pi }}}


so, {{{ (48*pi)/5 = (3y-9)*pi }}}
{{{ 48/5 = 3y-9 }}}
{{{ 48 = 5(3y-9) }}}
{{{ 48 = 15y-45 }}}
{{{ 93 = 15y }}}
--> y = 6.2



jon.