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NCERT Class XI Mathematics - Straight Lines - Solutions

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Question : 68 of 74
Marks: +1, -0
Find the distance of the line 4x + 7y + 5 = 0 from the point (1, 2) along the line 2x – y = 0.
Solution:  
The given equation of line is
2x – y = 0 ... (i)
and the point Q(1, 2) lies on this line.
We also have given
4x + 7y + 5 = 0 ... (ii)
On solving (i) and (ii), we get x = 518-\frac{5}{18} , y = 59-\frac{5}{9}
Thus the lines (i) and (ii) meet at R (518,59)\left(-\frac{5}{18}, -\frac{5}{9}\right)
∴ The required distance = |QR|
= (5181)2+(592)2\sqrt{ \left(-\frac{5}{18} - 1\right)^2 + \left(-\frac{5}{9} - 2\right)^2 }
= 529324+52981\sqrt{ \frac{529}{324} + \frac{529}{81} } = 52981(14+1)\sqrt{ \frac{529}{81} \left( \frac{1}{4} + 1 \right) } = 23954\frac{23}{9} \sqrt{ \frac{5}{4} } = 23518\frac{23 \sqrt{5}}{18} units.
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