Estimation: length 2 decimeter, width 1.5 decimeter, measurement: length 2.2 decimeter, width 1.6 decimeter,

Circumference: (2.2+ 1.6)×2=7.6 (decimeter);

(2) the length and width of the class desktop:

Estimation: length 1.5m, width 1m, measurement: length 1.6m, width 0.9m,

Circumference: (1.6+0.9)×2=5 (m);

Answer: The perimeter of the math textbook is 7.6 decimeters, and the perimeter of the desk is 5 meters.