Quadra Trigno Equation

If the roots of the quadratic equation x 2 + p x + q = 0 x^2 + px+ q = 0 are tan 3 0 and tan 1 5 \tan 30^ {\circ} \text {and} \tan 15^{\circ} , then the value of 2 + q p 2+q-p is?


The answer is 3.

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3 solutions

Vishal S
Jan 9, 2015

Given equation

x 2 x^{2} +px+q

Roots of the equation= tan 15 \tan 15 , tan 30 \tan 30

We know that

tan 15 \tan 15 =2- 3 \sqrt{3}

tan 30 \tan 30 = 1 3 \frac {1}{\sqrt{3}}

In the given equation

Sum of the roots = -p = 2- 3 \sqrt{3} + 1 3 \frac {1}{\sqrt{3}}

\Rightarrow p = 2 2 3 3 \frac {2-2\sqrt{3}}{\sqrt{3}}

Product of the roots = q=(2- 3 \sqrt{3} )( 1 3 \frac {1}{\sqrt{3}} )

\Rightarrow q= 2 3 3 \frac {2-\sqrt{3}}{\sqrt{3}}

Therefore 2+q-p = 2+ 2 3 3 \frac {2-\sqrt{3}}{\sqrt{3}} - 2 2 3 3 \frac {2-2\sqrt{3}}{\sqrt{3}}

\Rightarrow 2+ 3 3 \frac {\sqrt{3}}{\sqrt{3}}

\Rightarrow 2+1

\Rightarrow 3 \boxed{3}

You can use just tan(A+B)=tanA+tanB/1-tanAtanB tan(A+B)=tan(15+30)=1=(-p)/1-q so we get ans as 3 just by using sum of roots and product of roots concept.

mukesh jha - 6 years, 2 months ago
William Isoroku
Apr 13, 2015

The exact value of tan 15 = 2 3 \tan { 15 } =2-\sqrt { 3 } from the formula: tan ( α β ) = tan α tan β 1 + tan α tan β \tan { (\alpha -\beta) = } \frac { \tan { \alpha -\tan { \beta } } }{ 1+\tan { \alpha \tan { \beta } } } , where 15 = 45 30 15=45-30

The exact value of tan 30 = 3 3 \tan { 30 } =\frac { \sqrt { 3 } }{ 3 }

From Vieta's formulas, we know that:

tan 30 + tan 15 = p \tan { 30 } +\tan { 15 } =-p

tan 30 × tan 15 = q \tan { 30 } \times \tan { 15 } =q

So long story short, solve for p p and q q

Mukesh Jha
Apr 6, 2015

Quite an easy sum.So,I suppose no solution would be required for this one.

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