2, A-2 and A+2

3、 1 1、 13、 17、3 1、37、7 1、79、97

4, 18 and 8

5，A+B+C =235，A-B =80，A-C =90。 Replace b and c with a, B = A -80, C = A -90, and derive A = 135, B =55, C =45, and the greatest common factor.

6. Suppose that three numbers are A, B and C respectively. According to the meaning of the question, A+B+C = 365, 438+02, 7/A = 8/B = 9/C, it is deduced that 8A=7C, 8C=9B, 7C=9A, and b and c are replaced by a, b = 8a/7, and c =

7. The circumference is 265,438+060m. It takes 65,438+008 trees to plant a tree every 25 meters.