Answer:
I=\frac{t^2}{2}
Step-by-step explanation:
From exercise we have that x=t, t>0. Because A(t) be the area of the region in the first quadrant, we get that x started at 0. The limits for y are the following e-x and e. We get the integral :
I=\int\limits^0_t \int\limits^{e}_{e-x} 1 dy dx
I=\int\limits^0_t [y]_{e-x}^{e} dx
I=\int\limits^0_t (e-e+x) dx
I=\int\limits^0_t {x} \, dx
I=[\frac{x^2}{2} ]_{0}^{t}
I=\frac{t^2}{2}