It has two endpoints. A ray has one endpoint.
diameter
An angle is the union of two rays with a common endpoint.
Not necessarily - it depends on the geometry. The equator, on a sphere like the earth, is a straight line on a spherical surface. It has no endpoint.
Add the same amount again by finding the difference of the midpoint and end point. Example: If the end point is 3 and the mid point is 9. The difference between 3 and 9 is 6 so add 6 to 9 and get 15.
If you are only given one endpoint and a midpoint, you know what the middle of the line segment is. Since the midpoint is half of what the line segment's length is, all you have to do is find the distance between the endpoint given and the midpoint, then add that coordinate to your midpoint and get your other endpoint. For example: Endpoint A: (4,5) Midpoint: (6,8) Distance between: (2,3) Add (2,3) to (6,8) and get Endpoint B: (8,11).
double the length
Given only the midpoint you cannot.
Find the length of the segment with endpoints of (3,2) and (-3,-6).
-- Find the distance between the endpoint and the midpoint.-- Double that distance to get the length of the complete segment.-- When you're finished, sit quietly for a moment and ponder the meaning of "midpoint".
The answer depends on what information you have been given.
No, lines are ongoing, and have no endpoint.
If you mean endpoint (6, 9) and midpoint (7, 6) then the other endpoint is (8, 3)
to be honest I dont know
The direction of missing endpoint is the same as the direction from the known end point to the midpoint. The distance from the midpoint to the missing endpoint is the same as the distance from the known end point to the midpoint. In coordinate geometry it is simple. If the known end point is (p, q) and the mid point is (r, s) then the missing point is (2r - p, 2s - q).
There are two possible answers given the information. What isn't given is if the second point is one third of the way from the known or unknown endpoint. Say the known endpoint is (xe,ye) and the point one third of the way along is (xt,yt). If the point one third of the way is closest to the known endpoint, the other endpoint would be (xe+3*(xt-xe), ye+3*(yt-ye)). This is probably the answer implied by your question. If the point is closest to the unknown endpoint the the unknown endpoint is (xe+(3/2)*(xt-xe), ye+(3/2)*(yt-ye)).
The answer depends on what the endpoint is shared with!