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Quadratic Equation and Inequalities question

2021 · 24 Feb · Shift 1 · Q31
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  5. /2021 · 24 Feb · Shift 1 · Q31

Quadratic Equation and Inequalities question

2021 · 24 Feb · Shift 1 · Q31

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
Let p and q be two positive numbers such that p + q = 2 and p4+q4 = 272. Then p and q are roots of the equation :
  1. A
    x2 – 2x + 8 = 0
  2. B
    x2 - 2x + 136=0
  3. C
    x2 – 2x + 16 = 0
  4. D
    x2 – 2x + 2 = 0
View written solutionFree

Correct answer: C

  1. Interpret the given expression

The statement is intended as:

p+q=2andp4+q4=272.p+q=2 \quad \text{and} \quad p^4+q^4=272.p+q=2andp4+q4=272.

We need the quadratic equation whose roots are ppp and qqq.


  1. Use the identity for p4+q4p^4+q^4p4+q4

First find pqpqpq using

p2+q2=(p+q)2−2pq=4−2pq.p^2+q^2=(p+q)^2-2pq=4-2pq.p2+q2=(p+q)2−2pq=4−2pq.

Now,

p4+q4=(p2+q2)2−2p2q2.p^4+q^4=(p^2+q^2)^2-2p^2q^2.p4+q4=(p2+q2)2−2p2q2.

Substitute p2+q2=4−2pqp^2+q^2=4-2pqp2+q2=4−2pq:

p4+q4=(4−2pq)2−2(pq)2.p^4+q^4=(4-2pq)^2-2(pq)^2.p4+q4=(4−2pq)2−2(pq)2.

Given p4+q4=272p^4+q^4=272p4+q4=272, so

(4−2pq)2−2(pq)2=272.(4-2pq)^2-2(pq)^2=272.(4−2pq)2−2(pq)2=272.

Expand:

16−16pq+4p2q2−2p2q2=27216-16pq+4p^2q^2-2p^2q^2=27216−16pq+4p2q2−2p2q2=272 16−16pq+2p2q2=27216-16pq+2p^2q^2=27216−16pq+2p2q2=272 2p2q2−16pq−256=02p^2q^2-16pq-256=02p2q2−16pq−256=0 p2q2−8pq−128=0.p^2q^2-8pq-128=0.p2q2−8pq−128=0.

Let

t=pq.t=pq.t=pq.

Then

t2−8t−128=0.t^2-8t-128=0.t2−8t−128=0.

Solve:

t=8±64+5122=8±242.t=\frac{8\pm\sqrt{64+512}}{2}=\frac{8\pm 24}{2}.t=28±64+512​​=28±24​.

So,

t=16ort=−8.t=16 \quad \text{or} \quad t=-8.t=16ort=−8.

Since p,q>0p,q>0p,q>0, we must have pq>0pq>0pq>0, hence

pq=16.pq=16.pq=16.


  1. Form the quadratic equation with roots ppp and qqq

For roots ppp and qqq, the quadratic is

x2−(p+q)x+pq=0.x^2-(p+q)x+pq=0.x2−(p+q)x+pq=0.

Using

p+q=2,pq=16,p+q=2, \quad pq=16,p+q=2,pq=16,

we get

x2−2x+16=0.x^2-2x+16=0.x2−2x+16=0.


  1. Match with the options

The correct option is:

x2−2x+16=0\boxed{x^2-2x+16=0}x2−2x+16=0​

which is Option C.


  1. Comparison with stored answer

Stored correct answer: C

Our derived answer: C

So they agree.

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