why dont we use 4?

Ex : Let R be the region bounded by the graph of y =1+ x , y =3-x (y - axes, x - axes Sketch the region R and set up the integral that can be used find its area. solution: * points of intersection V1 = )2 3 - 1+vx =3-x chest 4-1 Vx =2-x square both sides why y = 1+ x 1 =4-4x +.x* den k x - -5x +4=0 X 2- y =]-x r = l 1=4X I Area = A, + A2 Area = [(1+ vx )dx + [(3-x )dx -1