(823-374)+(3477-3085)=449+392=841
12x-18+8-6x=5; 12x-6x=5-8+18; 6x=15; x=15/6=5/2=2,5.
((sin13a+sin15a)+(sin14a+sin16a))/((cos13a+cos15a)+(cos14a+cos16a))=
(2sin14a*cosa+2sin15a*cosa)/(2cos14acosa+2cos15acosa)=
=2cosa*(sin14a+sin15a)/2cosa(cos14a+cos15a)=
=(2sin29a/2*cosa/2)/(2cos29a/2*cosa/2)=tg(29a/2)