Hi, could I have help with just part b please, I need an answer that doesn't require the use of 3D vectors, 2D vector formulas etc are fine or the use of geometry without vectors but no 3D vectors please

Question 1 Figure 1 (a 2-dimensional diagram) shows a piece of paper in the shape of an isosceles triangle. AB = AC = a; AM L BC; LBAM = 0 Figure 2 (a 3-dimensional diagram) shows the paper folded along AM until the two halves are perpendicular. B B M M Figure 1 Figure 2 Show that, in Figure 2: (a) (BC)2 = 2a2sin20 [3] (b) if the angle formed by AB and AC is now p, cos p = cos20 [3]