Question 12? Choose c? 1/2 B 1A2= 1? B2A3=2？ B3A4=4？ B4A4=4？ B4A5=8……？ B6A7=32

Question 13? a(a2-b2)=？ A (A-B)? (a+b)

Positive solutions of 14 problem

Question 15? 3 pie /2? - 1? Large circle area reduces circle area? /4? — 1

Question 16? 7? Make an auxiliary line? Form a square AC=5? OCB angle =45 degrees