maths 趕！幫忙！10點！

1.Use a vector proof to show that the sum of the squares of diagonal lengths of a quadrilateral is equal to twice the sum of the squares of the lengths of the line segments joining the midpoints of opposite sides.

2. In the diagram (http://hk.geocities.com/sinyeeangela/DSCN5797_1.jpg) ABCD, is a square. M is the midpoint of DC, and AN 垂直 MB. Prove that DN=DA

thank you very much！幫幫忙！