S(CAB)= bc*sin(A)/2
S(CAD)= bl*sin(A/2)/2
S(BAD)= cl*sin(A/2)/2
S(CAB)=S(CAD)+S(BAD) =>
bc*sin(A)= bl*sin(A/2) + cl*sin(A/2) <=>
l= bc*sin(A)/(b+c)sin(A/2) =2bc*cos(A/2)/(b+c)
