Question 895428
Use Heron's formula.
{{{s=(1/2)(12+15+20)=47/2}}}
Then,
{{{A=sqrt(s(s-12)(s-15)(s-20))}}}
{{{A=sqrt((47/2)(47/2-12)(47/2-15)(47/2-20))}}}
{{{A=sqrt((47/2)(47/2-24/2)(47/2-30/2)(47/2-40/2))}}}
{{{A=sqrt((47/2)(23/2)(32/2)(7/2))}}}
{{{A=sqrt(128639/16)}}}
{{{A=sqrt(128639)/4)}}}