$$ \int_0^1 \int_0^1 (x+t) \, dx \, dy \nonumber $$ $$ \int_0^1 \left[ \frac{x^2}{2} +xy \right]_0^1 dy \nonumber $$ $$ \int_0^1 \left( \frac{y^2}{2} \right]_0^1 \nonumber $$ $$ \frac{1}{2}+\frac{1}{2}=1 \nonumber $$