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## Answers

1. sum of opposite angles is 180 deg.

2. a* c + b *d = e * f

This is Ptolemy's theorem.

3. The area of the quadrilateral is given by BrahmaGupta's theorem.

s = semiperimeter.

4. Angle ABD = angle ACD

5. Angle BCA = angle BDA

6. angle CAD = angle CBD

7, angle BAC = angle BDC