Topic: General types

Analysis: By observing the graphics, we can know that from the second pattern, adding a window will add three paper cuts. According to this rule, the number of paper-cuts in the n-th picture can be calculated.

Solution: Solution: The first pattern is 3+2=5 window grilles;

The second pattern is 2×3+2=8 window grilles;

The third pattern is 3×3+2= 1 1 window grillage;

... so that we can explore:

The number of bar grids of the nth pattern is (3n+2).

( 1)20

(2)3n+2

(3) Existence, let 3n+2=20 12, then 3n=20 10? n=670？ Therefore, it is the 670th.

Comments: This question is a regular question, which often appears in the senior high school entrance examination. For the problem of finding laws, we must first find out which parts have changed and according to what laws. The landlord hopes you are satisfied.