NCERT Solutions
Class 11 Maths
Straight Lines

Ex.Misc. Q.14
In what ratio, the line joining (–1, 1) and (5, 7) is divided by the line x + y = 4?
Given line is x + y = 4 ............. (1)
and points are: (-1, 1) and (5, 7)
Equation of line which joins the points
y - 1 = {(7 - 1) ÷ (5 + 1)} × (x + 1)
=> y-1 = (6 ÷ 6) × (x + 1)
=> y - 1 = x + 1
=> x - y + 1 + 1 = 0
=> x – y + 2 = 0 .......... (2)
After solving equation (1) and (2)
x = 1, y = 3
So, (1, 3) is the point of intersection of these two lines.
Now let point (1, 3) divide the line joining (-1, 1) and (5, 7) in the ratio 1: k
According to section formula,
(1, 3) = [{k (-1) + 1 × 5} ÷ (1 + k), (k × 1 + 1 × 7) ÷ (1 + k)]
=> (1, 3) = {(-k + 5) ÷ (1 + k), (k + 7) ÷ (1 + k)}
Now,
(-k + 5) ÷ (1 + k) = 1
=> -k + 5 = 1 + k
=> 5 - 1= k + k
=> 4 = 2k
=> k = 2
Hence, the required ratio is 1: 2