sin a+sin b=1/4, 2*sin((a+b)/2)*cos((a-b)/2)=1/4
cos a+cos b=1/3, 2*cos((a+b)/2)*cos((a-b)/2)=1/3
sin((a+b)/2)*cos((a-b)/2)=1/8 (1)
cos((a+b)/2)*cos((a-b)/2)=1/6 (2)
(1)/(2): tan((a+b)/2)=6/8=3/4
(a+b)/2=arc tan(3/4)=36.8699
a+b=73.7398
tan(a+b)=tan(73.7398)=3.42857