A rod of length eight units moves such that its ends A and B always lie on the lines and , respectively. If the locus of the point , that divides the rod internally in the ratio , is then is equal to:
- A
- B
- C
- D
A rod of length eight units moves such that its ends A and B always lie on the lines and , respectively. If the locus of the point , that divides the rod internally in the ratio , is then is equal to:
Correct answer:B
Standard Method
Given: End lies on and end lies on . The rod has fixed length units. Point divides internally in the ratio .
Find: from the locus of .
Take
since lies on and lies on .
Using the length of the rod,
so
Since divides internally in the ratio ,
Hence,
From , we get
And from ,
so
Now,
and
Substitute in the length condition:
Multiply by :
Divide by form equivalently to match the given locus after expansion.
Expanding,
Factor out from the required part:
Comparing with
we get
Therefore,
So, the correct option is B.
Using section formula and elimination
Given: lies on , lies on , and .
Find: The value of .
Write the moving points as
Then
If divides in the ratio , then
So
Substitute these into
This gives
Now simplify:
Expanding and collecting terms,
Hence,
Therefore,
and
Hence the correct option is B.
Using the wrong section formula. If divides internally in the ratio , the coordinates are weighted toward the opposite endpoint. Use consistently.
Writing the line condition for point incorrectly. From , we must get , not . A sign error here changes every coefficient in the locus.
Making an error in the distance formula. The vertical difference between and is , whose square is , not .
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