suppose G is a connected graph with n vertices and n-1 edges.
prove that G has at least 2 vertices with deg(v)=1 .
Then we know that where is the minimum vertex degree.
You can use that show that if then there must be at one vertex of degree one.
You are working with a connected graph. So all vertices are of degree alt least one.
You should know that .
Now you show some effort.