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 the roots of the quadratic equation $$x^2 + px + q = 0$$ are $$\tan 30^\circ$$ and $$\tan 15^\circ$$, respectively then the value of $$2 + q - p$$ is
Let the roots of the quadratic $$x^{2}+px+q=0$$ be $$r_{1}$$ and $$r_{2}$$.
Given: $$r_{1}=\tan 30^\circ$$ and $$r_{2}=\tan 15^\circ$$.
Step 1: Evaluate the numerical values of the roots.
For $$30^\circ$$ we know $$\tan 30^\circ = \frac{1}{\sqrt3}$$, so $$r_{1}=\frac{1}{\sqrt3}$$.
To get $$\tan 15^\circ$$, use the formula for $$\tan(A-B)$$:
$$\tan(A-B)=\frac{\tan A-\tan B}{1+\tan A\tan B}.$$
Choose $$A=45^\circ,\, B=30^\circ$$; thus
$$\tan 15^\circ = \tan(45^\circ-30^\circ) =\frac{\tan45^\circ-\tan30^\circ}{1+\tan45^\circ\tan30^\circ} =\frac{1-\frac{1}{\sqrt3}}{1+1\cdot\frac{1}{\sqrt3}}$$
Simplify numerator and denominator by multiplying top and bottom by $$\sqrt3$$:
$$\tan15^\circ
=\frac{\sqrt3-1}{\sqrt3+1}
=2-\sqrt3.$$
Hence $$r_{2}=2-\sqrt3$$.
Step 2: Relate coefficients to the roots.
For the quadratic $$x^{2}+px+q=0$$ with roots $$r_{1},r_{2}$$:
Sum of roots: $$r_{1}+r_{2}=-p.$$
Product of roots: $$r_{1}r_{2}=q.$$
Step 3: Compute $$r_{1}+r_{2}$$ and $$r_{1}r_{2}$$.
Sum:
$$r_{1}+r_{2}
=\frac{1}{\sqrt3}+2-\sqrt3
=\frac{\sqrt3}{3}+2-\sqrt3
=2-\frac{2\sqrt3}{3}.$$
Product:
$$r_{1}r_{2}
=\frac{1}{\sqrt3}\,(2-\sqrt3)
=\frac{2-\sqrt3}{\sqrt3}
=\frac{2}{\sqrt3}-1.$$
Thus
$$p=-(r_{1}+r_{2})= -\left(2-\frac{2\sqrt3}{3}\right),$$
$$q=r_{1}r_{2}= \frac{2}{\sqrt3}-1.$$
Step 4: Evaluate $$2+q-p$$.
First rewrite $$p$$ with a positive denominator: $$p=-2+\frac{2\sqrt3}{3}.$$
Now calculate:
$$$ \begin{aligned} 2+q-p &=2+\left(\frac{2}{\sqrt3}-1\right)-\left(-2+\frac{2\sqrt3}{3}\right)\\[4pt] &=2-1+2+\frac{2}{\sqrt3}-\frac{2\sqrt3}{3}\\[4pt] &=3+\left(\frac{2}{\sqrt3}-\frac{2\sqrt3}{3}\right). \end{aligned} $$$
Notice that $$\frac{2}{\sqrt3} = \frac{2\sqrt3}{3},$$ so the two radical terms cancel:
$$\frac{2}{\sqrt3}-\frac{2\sqrt3}{3}=0.$$
Therefore
$$2+q-p = 3.$$
Hence the required value is 3.
Option B which is: 3
Click on the Email ☝️ to Watch the Video Solution
Create a FREE account and get:
Educational materials for JEE preparation