Someone can help me to resolve this analysis 2 question? D stands for domain which corresponds to a triangle with the following vertices: (0,0) (0,1) (1,0)

is the following hypotesis correct?

D=((x,y)€R^2 : x>=0, y>=0, y<=-x+1)

u=y-x

v=y+x

=> y=(u+v)/2 and x=(v-u)/2

new domain D=>E=((u,v)€R^2 : v-u>=0, u+v>=0, v<=+1)

but i feel stuck..

tips?