1.

If (2, 0), (0, 1), (4, 5) and (0, c) are concyclic and then find c.

Answer»

x2 + y2 + 2gx + 2fy + c1 = 0 

Satisfies (2, 0), (0, 1) (4, 5) we get 

4 + 0 + 4g + c1 = 0 –– (i) 

0 + 1 + 2g. 0 + 2f + c1 = 0 –– (ii) 

16 + 25 + 8g + 10f + c1 = 0 –– (iii) 

(ii) – (i) we get 

– 3 – 4g + 2f = 0 

 4g – 2f = – 3 –– (iv) 

(ii) – (iii) we get 

– 40 – 8g – 8f = 0 

(or) g + f = – 5 –– (v) 

Solving(iv) and (v) we get 

g = -13/6, f = -17/6

Substituting g and f value in equation (i) we get 

4 + 4 (-13/6) + c1 = 0

c1 = 14/3

Now equation x2 + y2 – (13x/3) - (17y/3) + (14/3) = 0

 Now circle passes through (0, c) then

c2 - (17c/3) + (14/3) = 0

3c2 – 17c + 14 = 0 

⇒ (3c – 14) (c – 1) = 0 

(or) 

c = 1 or 14/3 



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