Use trigonometry

Algebra Level 3

A quadratic equation has products of roots equal to c. If the roots are of the form. (P^2 +Q^2)/(P^2+Q^2-PQ). And (P^2+Q^2)/(P^2+Q^2+PQ). Find maximum value of c P and Q are real


The answer is 1.33.

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3 solutions

Devansh Shah
Oct 20, 2015

P= asint. Q=acost. A is real Roots a^2(sin^2 t+cos^2 t)/a^2(sin^2 t+cos^2 t +sintcost)= First root=1/(1-sintcost) = 2/(2-sin2t) Second root= 2/(2+sin2t) Product of roots= 4/(4-sin^2 2t) Max value if c when sin^2 2t max which is 1 hence. Maximum value of c= 4/3 = 1.33

Dont be sad . I have upvoted your solution. But my dear friend since this is your first question on brilliant don't repeat the mistake

Aakash Khandelwal - 5 years, 7 months ago

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Mistake us you have not specified that p and q are real

Aakash Khandelwal - 5 years, 7 months ago

The only (other somewhat identical)solution using trigonometry

Aakash Khandelwal - 5 years, 7 months ago
Rahul Gupta
Oct 21, 2015

1.333333333333333333333333333333333333333333333333333333...

Aakash Khandelwal
Oct 20, 2015

Its simple algebra. And good application of AM>=GM. Simplifing c. We get c=1+ 1/(p/q)^2 + (q/p)^2 +1

but we know sum of a no. And its reciprocal cant exceed 2 if it is positive

Hence maximum value of c is 1.3333333333333333333333333333 ... As many times you can :)

Also in your solution specify that a is real

Aakash Khandelwal - 5 years, 7 months ago

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OK Aakash I edited my solution

Devansh Shah - 5 years, 7 months ago

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