Rotate the triangle APB 90 counterclockwise around A to get the triangle AP 1D== "Triangle AP 1D is a right triangle, AP = AP1= = > PP 1=√2, angle APP 1 = 45.
PP 1=√2,PD=√7,p 1D = 3 = = & gt; P1d 2 = PD 2+p1p 2 = = > triangle PP 1D is a right triangle.