an = 2+3n
bm = -1+4m m=1,2,..75
an =bm = 2+3n= -1+4m4m=3(n+1)
∵ （3,4）=1 ,∴ 3|mm=3k (k=1,2,...,25)
4m=4*3k=3(n+1)n=4k-1 (k=1,2,...,25)
cn = 2 + 3*(4k-1) = 12k-1 (k=1,2,...,25)