Prove that simple graph with n vertices and k components can have at most (n-k) (n-k+l)/2 edges.

To prove that a simple graph with n vertices and k components can have at most (n-k) (n-k+1)/2 edges, we can follow these steps:

Step 1: Understand the problem First, let's clarify what a simple graph is. A simple graph is an undirected graph with no loops or multiple edges between the same pair of vertices. In this case, we are considering a simple graph with n vertices and k components.

Step 2: Define components Components in a graph refer to the subgraphs that are disconnected from each other. In other words, a component is a group of vertices that are connected to each other but not to any other vertices in the graph.

Step 3: Determine the maximum number of edges To find the maximum number of edges in a simple graph with n vertices and k components, we need to consider the worst-case scenario. In this case, the worst-case scenario is when each component is disconnected from all other components.

Step 4: Calculate the number of edges in each component Since each component is disconnected from the others, we can calculate the number of edges in each component separately. For a component with m vertices, the maximum number of edges is given by (m-1) (m-1+1)/2 = m(m-1)/2.

Step 5: Sum up the number of edges in all components To find the maximum number of edges in the entire graph, we need to sum up the number of edges in each component. Since we have k components, the total number of edges is the sum of the number of edges in each component, which can be expressed as Σ(m(m-1)/2) for m ranging from 1 to k.

Step 6: Simplify the expression To simplify the expression, we can expand the summation and rearrange the terms. This gives us Σ(m(m-1)/2) = (1/2)Σ(m^2 - m) = (1/2)(Σm^2 - Σm).

Step 7: Calculate the summation of m^2 and m The summation of m^2 can be calculated using the formula Σm^2 = n(n+1)(2n+1)/6, and the summation of m can be calculated using the formula Σm = n(n+1)/2.

Step 8: Substitute the values into the expression Substituting the values of Σm^2 and Σm into the expression (1/2)(Σm^2 - Σm), we get (1/2)(n(n+1)(2n+1)/6 - n(n+1)/2).

Step 9: Simplify the expression further Simplifying the expression (1/2)(n(n+1)(2n+1)/6 - n(n+1)/2), we get (n(n+1)(2n+1)/12 - n(n+1)/4).

Step 10: Simplify the expression to the final form Further simplifying the expression (n(n+1)(2n+1)/12 - n(n+1)/4), we get (n(n+1)(2n+1) - 3n(n+1))/12 = (n(n+1)(2n+1 - 3))/12 = (n(n+1)(2n-2))/12 = n(n+1)(n-1)/6.

Step 11: Final result Therefore, the maximum number of edges in a simple graph with n vertices and k components is n(n+1)(n-1)/6, which can be further simplified to (n-k)(n-k+1)/2.

Thus, we have proven that a simple graph with n vertices and k components can have at most (n-k)(n-k+1)/2 edges.