The sum of the number of edges of a basic graph is that each edge is cut in half, that is, 2n; The sum of the edges of an ultra-small graph is how many polygons the basic graph has, so it has several small parts with one edge less than the basic graph, that is, n(n- 1).
So the formula an = 2n+n (n-1) = n (n+1) ... A5 = 5 * 6 = 30.
The second space is calculated according to the formula:1/a3+1/a4+1/a5+1/an =197/600.
That is 1/3*4.
+
1/4*5
+
1/5*6
+
1/n(n+ 1)= 197/600
First, 1/n(n+ 1) can be extended to
= 1+n-n/n(n+ 1) becomes about.
= 1/n- 1/(n+ 1)
Then it is 1/3*4.
+
1/4*5
+
1/5*6
+
1/n (n+1) =197/600 becomes
1/3- 1/4
+
1/4- 1/5
+
1/5- 1/6
+
1/n- 1/(n+ 1)= 197/600
The final result is1/3-1/(n+1) =197/600.
n= 199
Okay, okay, can you understand? Hehe.
It's tiring to look at it this way. Write it on paper once and you can understand it.
Very simple