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Circle question

2022 · 25 Jun · Shift 1 · Q41
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  5. /2022 · 25 Jun · Shift 1 · Q41

Circle question

2022 · 25 Jun · Shift 1 · Q41

JEE MainMathematicsCircleNumerical+4 / −1
Let the abscissae of the two points P and Q be the roots of 2x2−rx+p=02{x^2} - rx + p = 02x2−rx+p=0 and the ordinates of P and Q be the roots of x2−sx−q=0{x^2} - sx - q = 0x2−sx−q=0. If the equation of the circle described on PQ as diameter is 2(x2+y2)−11x−14y−22=02({x^2} + {y^2}) - 11x - 14y - 22 = 02(x2+y2)−11x−14y−22=0, then 2r+s−2q+p2r + s - 2q + p2r+s−2q+p is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 7

Let P=(x1,y1), Q=(x2,y2).P=(x_1,y_1),\, Q=(x_2,y_2).P=(x1​,y1​),Q=(x2​,y2​).

We are given:

  • the abscissae x1,x2x_1,x_2x1​,x2​ are roots of 2x2−rx+p=0,2x^2-rx+p=0,2x2−rx+p=0,
  • the ordinates y1,y2y_1,y_2y1​,y2​ are roots of x2−sx−q=0.x^2-sx-q=0.x2−sx−q=0.

The circle with diameter PQPQPQ is 2(x2+y2)−11x−14y−22=0.2(x^2+y^2)-11x-14y-22=0.2(x2+y2)−11x−14y−22=0.


1. Write the circle in standard form

Divide by 222: x2+y2−112x−7y−11=0.x^2+y^2-\frac{11}{2}x-7y-11=0.x2+y2−211​x−7y−11=0.

Compare with the general equation of a circle having diameter endpoints (x1,y1)(x_1,y_1)(x1​,y1​) and (x2,y2)(x_2,y_2)(x2​,y2​): x2+y2−(x1+x2)x−(y1+y2)y+x1x2+y1y2=0.x^2+y^2-(x_1+x_2)x-(y_1+y_2)y+x_1x_2+y_1y_2=0.x2+y2−(x1​+x2​)x−(y1​+y2​)y+x1​x2​+y1​y2​=0.

So we get x1+x2=112,y1+y2=7,x1x2+y1y2=−11.x_1+x_2=\frac{11}{2}, \qquad y_1+y_2=7, \qquad x_1x_2+y_1y_2=-11.x1​+x2​=211​,y1​+y2​=7,x1​x2​+y1​y2​=−11.


2. Use Vieta's formulas for the abscissae

For 2x2−rx+p=0,2x^2-rx+p=0,2x2−rx+p=0, the sum and product of roots are x1+x2=r2,x1x2=p2.x_1+x_2=\frac{r}{2}, \qquad x_1x_2=\frac{p}{2}.x1​+x2​=2r​,x1​x2​=2p​.

Since x1+x2=112x_1+x_2=\frac{11}{2}x1​+x2​=211​, r2=112  ⟹  r=11.\frac{r}{2}=\frac{11}{2} \implies r=11.2r​=211​⟹r=11.


3. Use Vieta's formulas for the ordinates

For x2−sx−q=0,x^2-sx-q=0,x2−sx−q=0, the sum and product of roots are y1+y2=s,y1y2=−q.y_1+y_2=s, \qquad y_1y_2=-q.y1​+y2​=s,y1​y2​=−q.

Since y1+y2=7y_1+y_2=7y1​+y2​=7, s=7.s=7.s=7.


4. Use the constant term of the circle

We have x1x2+y1y2=−11.x_1x_2+y_1y_2=-11.x1​x2​+y1​y2​=−11.

Now x1x2=p2,y1y2=−q.x_1x_2=\frac{p}{2}, \qquad y_1y_2=-q.x1​x2​=2p​,y1​y2​=−q.

Hence p2−q=−11.\frac{p}{2}-q=-11.2p​−q=−11. Multiply by 222: p−2q=−22.p-2q=-22.p−2q=−22.


5. Find the required expression

We need 2r+s−2q+p.2r+s-2q+p.2r+s−2q+p.

Substitute r=11r=11r=11, s=7s=7s=7, and p−2q=−22p-2q=-22p−2q=−22: 2r+s−2q+p=2(11)+7+(p−2q)=22+7−22=7.2r+s-2q+p=2(11)+7+(p-2q)=22+7-22=7.2r+s−2q+p=2(11)+7+(p−2q)=22+7−22=7.


Final Answer

7\boxed{7}7​

The derived answer matches the stored correct answer.

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