Example 1 :
In what ratio does the point P(-2, 3) divide the line segment joining the points A(-3, 5) and B (4, -9) internally?
Solution :
Given points are (-3 , 5) and B (4 ,- 9).
Let P (-2, 3) divide AB internally in the ratio l : m.
By the section formula,
[(lx2 + mx1)/(l + m), (ly2 + my1)/(l + m)] = (-2, 3)
Substitute (x1, y1) = (-3, 5) and (x2, y2) = (4, -9).
(l(4) + m(-3))/(l+m), (l(-9) + m(5))/(l+m) = (-2, 3)
(4l - 3m)/(l+m), (-9l + 5m)/(l+m) = (-2, 3)
Equating the coefficients of x, we get
(4l - 3m)/(l+m) = -2
4l - 3m = -2(l + m)
4l - 3m = -2l - 2m
Add 2l and 3m on both sides
6l = m
l/m = 1/6
l : m = 1 : 6
Hence the point P divides the line segment joining the points in the ratio 1 : 6.
Example 2 :
Let A (-6, -5) and B (-6, 4) be two points such that a point P on the line AB satisfies AP = (2/9)AB. Find the point P.
Solution :
AP = (2/9)AB
9AP = 2AB
9AP = 2(AP + PB)
9AP = 2AP + 2PB
9AP – 2AP = 2PB
7AP = 2PB
AP/AB = 2/7
AP: PB = 2 : 7
So P divides the line segment in the ratio 2 : 7.
Section formula internally
= [(lx2 + mx1)/(l + m), (ly2 + my1)/(l + m)]
Substitute l = 2 and m = 7.
= [(2(-6)) + (7(-6)]/(2+7), [(2(4)) + (7(-5)]/(2+7)
= [(-12-42)/9, (8 - 35)/9]
= (-54/9, -21/7)
= (-6, -3)
Example 3 :
Find the ratio in which the x-axis divides the line segment joining the points (6, 4) and (1, -7).
Solution :
Let l : m be the ratio of the line segment joining the points (6, 4) and (1, -7) and let p(x, 0) be the point on the x axis.
= (lx2 + mx1)/(l + m), (ly2 + my1)/(l + m)
(x, 0) = [l(1) + m(6)]/(l + m), [l(-7) + m(4)]/(l + m)
(x, 0) = [l + 6 m]/(l + m), [-7l + 4m]/(l + m)
Equating y-coordinates, we get
[-7l + 4m]/(l + m) = 0
-7l + 4m = 0
-7l = -4m
l/m = 4/7
l : m = 4 : 7
Hence x-axis divides the line segment in the ratio 4 : 7.
Example 4 :
In what ratio is the line joining the points (-5, 1) and (2, 3) divided by y-axis? Also, find the point of intersection.
Solution :
Let L : m be the ratio of the line segment joining the points (-5 , 1) and (2 ,3) and let p(0,y) be the point on the y axis
Section formula internally
= (lx2 + mx1)/(l + m) , (ly2 + my1)/(l + m)
(0 , y) = [L(2) + m(-5)]/(L + m) , [L(3) + m(1)]/(L + m)
(0 , y) = [2L - 5 m]/(L + m) , [3L + m]/(L + m)
[2L - 5 m]/(L + m) = 0
2 l - 5 m = 0
2 l = 5 m
l/m = 5/2
l : m = 5 : 2
To find the required point we have to apply this ratio in the formula
(0, y) = [2(5) – 5(2)]/(5 + 2), [3(5) + 2]/(5 + 2)
(0 , y) = [10 – 10]/7, [15 + 2]/7
(0 , y) = (0 , 17/7)
Hence the required point is (0, 17/7).
Example 5 :
If (a/3, 4) is the midpoint of the line segment joining the points Q(-6, 5) and R(-2, 3), then find the value of a.
Solution :
It can be done using section formula or midpoint formula. Since the point (a/3, 4) is the midpoint, the line segment divided by this point will be in the ratio of 1 : 1 .
Midpoint = (x1 + x2)/2, (y1 + y2)/2
(-6 + (-2))/2, (5 + 3)/2 = (a/3, 4)
(-8)/2, 8/2 = (a/3, 4)
(-4, 4) = (a/3, 4)
Equating the x and y-coordinates, we get
a/3 = -4
a = -12
So, the value of a is -12.
Example 6 :
A line intersects y-axis and x-axis at the points P and Q respectively. If (2, -5) is the midpoint of PQ, then find the coordinates of P and Q respectively.
Solution :
Let (x, 0) be the point of x-intercept
Let (0, y) be the point of y-intercept.
(x + 0)/2, (0 + y)/2 = (2, -5)
(x/2, y/2) = (2, -5)
x/2 = 2 and y/2 = -5
x = 4 and y = -10
So, the required points are Q(4, 0) and P(0, -10).
Example 7 :
Find the rational number which y-axis divides the line segment joining (-3, 6) and (12, -3).
Solution :
Let (0, y) be the point on y-axis and l : m is the required ratio
(lx2 + mx1)/(l + m), (ly2 + my1)/(l + m) = (0, y)
(12l - 3m)/(l + m), (-3l + 6m)/(l + m) = (0, y)
12l - 3m = 0
12l - 3m = 0
3m = 12l
3/12 = l/m
1/4 = l/m
l : m is 1 : 4.
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