a(3)= a( 1)+a(2)= 6；
a(4)= a(2)+a(3)= 9；
a(5)=a(3)+a(4)= 15 .
To sum up, the law is like this
a(n)=a(n-2)+a(n- 1).
therefore
a(6)= a(4)+a(5)= 24；
a(7)= a(5)+a(6)= 39；
a(8)=a(6)+a(7)=63 .
A: The order of filling is 3, 3, 6, 9, 15, (24), (39) and (63).
Difficulties: How to determine the point M? Because the side lengths of the left right-angled triangle are 3, 4 and 5 respective