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Quadratic Equation and Inequalities question

2018 · 16 Apr · Shift 1 · Q40
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  5. /2018 · 16 Apr · Shift 1 · Q40

Quadratic Equation and Inequalities question

2018 · 16 Apr · Shift 1 · Q40

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
If an angle A of a Δ\DeltaΔ ABC satiesfies 5 cosA + 3 = 0, then the roots of the quadratic equation, 9x2 + 27x + 20 = 0 are :
  1. A
    secA, cotA
  2. B
    sinA, secA
  3. C
    secA, tanA
  4. D
    tanA, cosA
View written solutionFree

Correct answer: C

  1. Given condition on the angle

We have 5cos⁡A+3=05\cos A + 3 = 05cosA+3=0 So, cos⁡A=−35\cos A = -\frac{3}{5}cosA=−53​

Since AAA is an angle of a triangle, 0<A<π0<A<\pi0<A<π. Also cos⁡A<0\cos A<0cosA<0, so AAA is obtuse and hence sin⁡A>0\sin A>0sinA>0.

Therefore, sin⁡A=1−cos⁡2A=1−925=1625=45\sin A = \sqrt{1-\cos^2 A} = \sqrt{1-\frac{9}{25}}=\sqrt{\frac{16}{25}}=\frac{4}{5}sinA=1−cos2A​=1−259​​=2516​​=54​

Now compute the required trigonometric values: sec⁡A=1cos⁡A=−53\sec A = \frac{1}{\cos A} = -\frac{5}{3}secA=cosA1​=−35​ tan⁡A=sin⁡Acos⁡A=4/5−3/5=−43\tan A = \frac{\sin A}{\cos A} = \frac{4/5}{-3/5} = -\frac{4}{3}tanA=cosAsinA​=−3/54/5​=−34​ cot⁡A=1tan⁡A=−34\cot A = \frac{1}{\tan A} = -\frac{3}{4}cotA=tanA1​=−43​


  1. Solve the quadratic equation

Given: 9x2+27x+20=09x^2+27x+20=09x2+27x+20=0

Factorize: 9x2+27x+20=9x2+15x+12x+209x^2+27x+20 = 9x^2+15x+12x+209x2+27x+20=9x2+15x+12x+20 =3x(3x+5)+4(3x+5)=3x(3x+5)+4(3x+5)=3x(3x+5)+4(3x+5) =(3x+5)(3x+4)=(3x+5)(3x+4)=(3x+5)(3x+4)

So the roots are x=−53,  −43x=-\frac{5}{3},\; -\frac{4}{3}x=−35​,−34​


  1. Match with trigonometric values

We found: sec⁡A=−53,tan⁡A=−43\sec A=-\frac{5}{3}, \qquad \tan A=-\frac{4}{3}secA=−35​,tanA=−34​

These are exactly the roots of the quadratic equation.

So the roots are: sec⁡A,  tan⁡A\sec A,\; \tan AsecA,tanA


  1. Check options
  • A: sec⁡A,cot⁡A=−53,−34\sec A, \cot A = -\frac{5}{3}, -\frac{3}{4}secA,cotA=−35​,−43​ ❌
  • B: sin⁡A,sec⁡A=45,−53\sin A, \sec A = \frac{4}{5}, -\frac{5}{3}sinA,secA=54​,−35​ ❌
  • C: sec⁡A,tan⁡A=−53,−43\sec A, \tan A = -\frac{5}{3}, -\frac{4}{3}secA,tanA=−35​,−34​ ✅
  • D: tan⁡A,cos⁡A=−43,−35\tan A, \cos A = -\frac{4}{3}, -\frac{3}{5}tanA,cosA=−34​,−53​ ❌

Hence, the correct option is C.

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