[sin^2(x) - 3*cos^2(x)]/[2*sin^2(x) + cos^2(x) ] = U
tg(x)=3,
Разделим числитель и знаменатель на cos^2(x), имеем
U = [ ( sin^2(x)/cos^2(x) ) -3 ]/[ 2*(sin^2(x)/cos^2(x) ) + 1 ],
т.к. sin(x)/cos(x) = tg(x), тогда (sin^2(x)/cos^2(x)) = tg^2(x),
U = [ tg^2(x) - 3 ]/[ 2*tg^2(x) +1] = [ 3^2 -3]/[2*3^2+1] =
= [9-3]/[2*9+1] = 6/19