Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
If an angle A of a $$\triangle ABC$$ satisfies $$5\cos A + 3 = 0$$, then the roots of the quadratic equation $$9x^2 + 27x + 20 = 0$$ are:
We are told that in $$\triangle ABC$$ the angle $$A$$ satisfies the relation $$5\cos A + 3 = 0$$.
First, we isolate $$\cos A$$:
$$5\cos A + 3 = 0 \;\Longrightarrow\; 5\cos A = -3 \;\Longrightarrow\; \cos A = -\dfrac{3}{5}.$$
Since $$A$$ is an angle of a triangle, we know $$0 < A < \pi$$. The value $$\cos A = -\dfrac{3}{5}$$ is negative, so $$A$$ lies in the second quadrant, where sine is positive and cosine is negative.
Now we compute $$\sin A$$ using the fundamental Pythagorean identity $$\sin^2\!A + \cos^2\!A = 1$$.
$$\sin^2\!A = 1 - \cos^2\!A = 1 - \left(-\dfrac{3}{5}\right)^2 = 1 - \dfrac{9}{25} = \dfrac{25}{25} - \dfrac{9}{25} = \dfrac{16}{25}.$$
Because $$A$$ is in the second quadrant, $$\sin A$$ is positive, hence
$$\sin A = \dfrac{4}{5}.$$
Next we find the other required trigonometric ratios.
The secant is the reciprocal of cosine, so
$$\sec A = \dfrac{1}{\cos A} = \dfrac{1}{-\dfrac{3}{5}} = -\dfrac{5}{3}.$$
The tangent is the quotient of sine by cosine, so
$$\tan A = \dfrac{\sin A}{\cos A} = \dfrac{\dfrac{4}{5}}{-\dfrac{3}{5}} = -\dfrac{4}{3}.$$
Now we examine the quadratic equation
$$9x^2 + 27x + 20 = 0$$
and test whether $$x = \sec A$$ or $$x = \tan A$$ satisfy it.
Substituting $$x = \sec A = -\dfrac{5}{3}$$:
$$9\left(-\dfrac{5}{3}\right)^2 + 27\left(-\dfrac{5}{3}\right) + 20 = 9\left(\dfrac{25}{9}\right) + 27\left(-\dfrac{5}{3}\right) + 20 = 25 - 45 + 20 = 0.$$
So $$x = \sec A$$ is indeed a root.
Substituting $$x = \tan A = -\dfrac{4}{3}$$:
$$9\left(-\dfrac{4}{3}\right)^2 + 27\left(-\dfrac{4}{3}\right) + 20 = 9\left(\dfrac{16}{9}\right) + 27\left(-\dfrac{4}{3}\right) + 20 = 16 - 36 + 20 = 0.$$
Thus $$x = \tan A$$ is also a root.
Since the two roots of the quadratic are $$\sec A$$ and $$\tan A$$, they correspond exactly to Option B.
Hence, the correct answer is Option B.
Create a FREE account and get:
Educational materials for JEE preparation