据每个数是它下一行左右相邻两数的和,先求出第7,8,9三行的第2个数,再求出8,9两行的第3个数,求出第9行的第4个数.
【解析】
设第n行第m个数为a(n,m),
由题意知a(6,1)=,a(7,1)=,a(8,1)=,a(9,1)=
∴a(7,2)=a(6,1)-a(7,1)=,a(8,2)=a(7,1)-a(8,1)=,a(9,2)=a(8,1)-a(9,1)=,
a(8,3)=a(7,2)-a(8,2)=,a(9,3)=a(8,2)-a(9,2)=
a(9,4)=a(8,3)-a(9,3)=
故选C