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Tangent to the curve y = x2 + 6 at a point (1, 7) touches the circle x2 + y2 + 16x + 12y + c = 0 at a point Q. Then the coordinates of Q are (2005S)
  • a)
    (–6, –11)
  • b)
    (–9, –13)
  • c)
    (–10, –15)
  • d)
    (–6, –7)
Correct answer is option 'D'. Can you explain this answer?
Verified Answer
Tangent to the curve y = x2 + 6 at a point (1, 7) touches the circle x...
The given curve is y = x2 + 6 Equation of tangent at (1, 7) is (y + 7) = x .1 + 6
⇒ 2x – y + 5 = 0 ...(1)
As given this tangent (1) touches the circle x2 + y2 +16x + 12y + c = 0 at Q
Centre of circle = (– 8, – 6).
Then equation of CQ which is perpendicular to (1) and passes through (– 8, – 6) is y + 6 = -(x + 8)
⇒ x + 2y + 20 = 0 ...(2)
Now Q is pt. of intersection of (1) and (2)
∴   Solving eqn (1) & (2) we get x = – 6, y = – 7
∴ Req. pt. is (– 6, – 7).
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Most Upvoted Answer
Tangent to the curve y = x2 + 6 at a point (1, 7) touches the circle x...
To find the equation of the tangent to the curve y = x^2 - 6 at the point (1,7), we need to find the slope of the tangent at that point. We can do this by taking the derivative of the function:

y' = 2x

At x=1, the slope of the tangent is y'(1) = 2.

So the equation of the tangent is:

y - 7 = 2(x - 1)

Simplifying, we get:

y = 2x + 5

To find the point where this tangent touches the circle x^2 + y^2 - 16x - 12y + c = 0, we substitute y = 2x + 5 into the equation of the circle:

x^2 + (2x+5)^2 - 16x - 12(2x+5) + c = 0

Simplifying, we get:

5x^2 - 72x + c - 107 = 0

We know that the tangent touches the circle at only one point, so the discriminant of this quadratic equation must be zero:

72^2 - 4(5)(c-107) = 0

Solving for c, we get:

c = 2005

So the equation of the circle is:

x^2 + y^2 - 16x - 12y + 2005 = 0

To find the point where the tangent touches the circle, we substitute y = 2x + 5 into the equation of the circle and solve for x:

x^2 + (2x+5)^2 - 16x - 12(2x+5) + 2005 = 0

Simplifying, we get:

5x^2 - 72x + 2005 = 0

Using the quadratic formula, we get:

x = (72 ± √(72^2 - 4(5)(2005))) / (2(5))

x = (72 ± 34√21) / 5

Since the tangent touches the circle at only one point, we choose the solution that is farther away from the point (1,7):

x = (72 + 34√21) / 5

Now we can find y by substituting this value of x into y = 2x + 5:

y = 2((72 + 34√21) / 5) + 5

Simplifying, we get:

y = 144/5 + 68√21/5

So the coordinates of the point where the tangent touches the circle are:

Q = ((72 + 34√21) / 5, 144/5 + 68√21/5)

Therefore, the answer is (2005S) b).
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Tangent to the curve y = x2 + 6 at a point (1, 7) touches the circle x2 + y2 + 16x + 12y + c = 0 at a point Q. Then the coordinates of Q are (2005S)a)(–6, –11)b)(–9, –13)c)(–10, –15)d)(–6, –7)Correct answer is option 'D'. Can you explain this answer?
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