Connecting Two Lines with a Rounded Corner

Click Create Rounded Corner from the Sketch group on the Design tab.
Select a line or arc.The first line or arc you select will be extended, if necessary.
Hover over another line to preview the round that will be created.If you hover over a line that is the edge of a solid, that edge will not change. Only a sketched line will change. The radius of the arc is previewed as you move the mouse. Specify a radius to dimension it.
Select an intersecting line to trim the lines, or a non-intersecting line to connect the lines.Where you click the intersecting line defines the radius of the tangent arc, or you can dimension the round (or chamfer) by entering its diameter.If the lines intersect, click the part of the intersecting line you want to keep. If the lines do not intersect, you can click anywhere on the line to extend the first line to form a rounded corner. Circles are never trimmed.

For lines that meet at a corner, you can select and drag the vertex.

You can also select the tool first and then make your selection.