3/ cosxcosy-sinxsiny=cos(x+y)
cos24cos31-sin24sin31=cos(24+31)=cos(55)
sinxcosy+cosxsiny=sin(x+y)
sin38cos12+cos38sin12=sin(38+12)=sin(50)
2/ log3 x- log9 x+log81 x=3/4
log3 x - log3^2 x + log 3^4 x =3/4
log3 x -1/2 log3 x + 1/4 log x=3/4
log3 x (1-1/2+1/4)=3/4
3/4 log3 x=3/4
log3 x=1
x=3
1-й сами поучите