As shown in Figure 8, fold triangle ABC along De, when point a falls inside the quadrilateral BCDE As shown in the figure, when the triangle paper ABC is folded along De, when point a falls inside the quadrilateral BCDE, there is a quantitative relationship between a and ∠ 1 ＋ 2, which always remains unchanged. Please find out this rule? And write down the reasons

As shown in Figure 8, fold triangle ABC along De, when point a falls inside the quadrilateral BCDE As shown in the figure, when the triangle paper ABC is folded along De, when point a falls inside the quadrilateral BCDE, there is a quantitative relationship between a and ∠ 1 ＋ 2, which always remains unchanged. Please find out this rule? And write down the reasons

Draw the triangle a'ed corresponding to the triangle AED
^A = ^ a = a ', angle AED = angle a'ed
Because ^ 1 + 2 ^ AED = 180 ° ^ 2 + 2 ^ EDA = 180 °
So ^ 1 + ^ 2 + 2 ^ AED + 2 ^ EDA = 360 degree
Because ^ A + ^ AED + ^ EDA = 108 degrees
So ^ AED + ^ EDA = 180 ° - ^ A
So ^ 1 + ^ 2 + 2 (180 ° - ^ a) = 360 °
^1+^2=2^A