NCERT Class XI Mathematics - Conic Sections - Solutions
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Question : 12
Total: 71
Find the equation of the circle with radius 5 whose centre lies on x-axis and passes through the point (2, 3).
Solution:
Since the centre of the circle lies on x-axis, the co-ordinates of centre are (h, 0).
Now the circle passes through the point (2, 3).
∴ Radius of circle
=√ ( 2 − h ) 2 + ( 3 − 0 ) 2 = √ h 2 + 4 − 4 h + 9 = √ h 2 − 4 h + 13
But radius of circle = 5
∴√ h 2 − 4 h + 13 = 5
⇒h 2 – 4h + 13 = 25 ⇒ h 2 – 4h – 12 = 0
⇒ (h – 6)(h + 2) = 0 ⇒ h = 6 or h = –2
When h = 6
Equation of circle is( x – 6 ) 2 + ( y – 0 ) 2 = ( 5 ) 2
⇒x 2 + 36 – 12x + y 2 = 25 ⇒ x 2 + y 2 – 12x + 11 = 0
When h = –2
Equation of circle is( x + 2 ) 2 + ( y – 0 ) 2 = ( 5 ) 2
⇒x 2 + 4 + 4x + y 2 = 25 ⇒ x 2 + y 2 + 4x – 21 = 0.
Now the circle passes through the point (2, 3).
∴ Radius of circle
=
But radius of circle = 5
∴
⇒
⇒ (h – 6)(h + 2) = 0 ⇒ h = 6 or h = –2
When h = 6
Equation of circle is
⇒
When h = –2
Equation of circle is
⇒
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