Q116 / 180 · COMEDK 2021 · Mathematics
The slopes of the lines, which make an angle $45^{\circ}$ with the line $3 x-y=-5$, are
- A$1,-1$
- B$\dfrac{1}{2},-1$
- C$1, \dfrac{1}{2}$
- Correct answerD$-2, \dfrac{1}{2}$
Solution
The given line is $3x - y = -5$, which can be written in the slope-intercept form $y = 3x + 5$.
The slope of this line is $m_1 = 3$.
Let the slope of the required lines be $m$. The angle between the lines is given as $45^{\circ}$.
Using the formula for the angle between two lines, $\tan \theta = \left| \dfrac{m - m_1}{1 + m m_1} \right|$, we have:
$\tan 45^{\circ} = \left| \dfrac{m - 3}{1 + 3m} \right|$
$1 = \left| \dfrac{m - 3}{1 + 3m} \right|$
This leads to two cases:
Case 1: $\dfrac{m - 3}{1 + 3m} = 1$
$m - 3 = 1 + 3m$
$-2m = 4 \Rightarrow m = -2$
Case 2: $\dfrac{m - 3}{1 + 3m} = -1$
$m - 3 = -1 - 3m$
$4m = 2 \Rightarrow m = \dfrac{1}{2}$
The slopes of the lines are $-2$ and $\dfrac{1}{2}$.
Answer: $-2, \dfrac{1}{2}$
The slope of this line is $m_1 = 3$.
Let the slope of the required lines be $m$. The angle between the lines is given as $45^{\circ}$.
Using the formula for the angle between two lines, $\tan \theta = \left| \dfrac{m - m_1}{1 + m m_1} \right|$, we have:
$\tan 45^{\circ} = \left| \dfrac{m - 3}{1 + 3m} \right|$
$1 = \left| \dfrac{m - 3}{1 + 3m} \right|$
This leads to two cases:
Case 1: $\dfrac{m - 3}{1 + 3m} = 1$
$m - 3 = 1 + 3m$
$-2m = 4 \Rightarrow m = -2$
Case 2: $\dfrac{m - 3}{1 + 3m} = -1$
$m - 3 = -1 - 3m$
$4m = 2 \Rightarrow m = \dfrac{1}{2}$
The slopes of the lines are $-2$ and $\dfrac{1}{2}$.
Answer: $-2, \dfrac{1}{2}$
Hence, the correct answer is option: 'D'.