D:
0<x<π /4
0 < y < tgx
S= ∫∫_(D)dxdy= ∫ _(0)^(π/4)[blue]( ∫_(0) ^(tgx)dy)[/blue]dx=∫ _(0)^(π/4)[blue](y)|_(0) ^(tgx))[/blue]dx=
=∫ _(0)^(π/4)([blue]tgx[/blue])dx=(-ln|cosx|)|_(0)^(π/4)=-ln cos(π/4)+lncos0=-ln(sqrt(2)/2)+0=-ln2^((-1/2))=[b](1/2)*ln2[/b]