2009 Hong kong National Olympiad

1

let an be a sequence of integers,a1 is odd,and for any positive integer n,we have n(an+1an+3)=an+1+an+3,in addition,we have 2010 divides a2009 find the smallest n 2,so that 2010 divides an

2

there are n points on the plane,any two vertex are connected by an edge of red,yellow or green,and any triangle with vertex in the graph contains exactly 2 colours.prove that n<13

3

ABC is a right triangle with C=90,CD is perpendicular to AB,and D is the foot,ω is the circumcircle of triangle BCD,ω1 is a circle inside triangle ACD,tangent to AD and AC at M and N respectively,and ω1 is also tangent to ω.prove that: (1)BDCN+BCDM=CDBM (2)BM=BC

4

find all pairs of non-negative integer pairs (m,n),satisfies 107^{56}(m^{2}-1)+2m+3=\binom{113^{114}}{n}