Coordinate Geometry | Exercise 7.2

Question 5

Find the ratio in which the line segment joining A(1,5)A(1, -5) and B(4,5)B(-4, 5) is divided by the xx-axis. Also find the coordinates of the point of division.

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Solution
Understand the Question
  • Any point on the xx-axis has coordinates of the form (x,0)(x, 0), meaning its yy-coordinate is always 00.
  • By the section formula, the coordinates of a point P(x,y)P(x, y) dividing the segment joining A(x1,y1)A(x_1, y_1) and B(x2,y2)B(x_2, y_2) in the ratio k:1k : 1 are: P(x,y)=(kx2+x1k+1,  ky2+y1k+1)P(x, y) = \left(\dfrac{k x_2 + x_1}{k + 1}, \; \dfrac{k y_2 + y_1}{k + 1}\right)
  • Setting the yy-coordinate equal to 00 gives the value of kk, which gives the ratio. Substituting kk back into the formula gives the xx-coordinate of the point of division.

Step 1 · Find the Ratio

Let the point of division on the xx-axis be P(x,0)P(x, 0), and let it divide the line segment joining A(1,5)A(1, -5) and B(4,5)B(-4, 5) in the ratio k:1k : 1.Using the section formula for the yy-coordinate:

y=ky2+1y1k+10=k(5)+1(5)k+10=5k5k+10×(k+1)=5k55k=5k=1\begin{aligned} y &= \dfrac{k y_2 + 1 y_1}{k + 1} \\[0.6em] 0 &= \dfrac{k(5) + 1(-5)}{k + 1} \\[0.6em] 0 &= \dfrac{5k - 5}{k + 1} \\[0.6em] 0 \times (k + 1) &= 5k - 5 \\ 5k &= 5 \\ k &= 1 \end{aligned}

Therefore, the xx-axis divides the line segment in the ratio 1:11 : 1 (i.e., PP is the midpoint of ABAB).

Step 2 · Find the Coordinates of the Point of Division

Substitute k=1k = 1 into the section formula for the xx-coordinate:

x=kx2+1x1k+1=1(4)+1(1)1+1=4+12=32\begin{aligned} x &= \dfrac{k x_2 + 1 x_1}{k + 1} \\[0.6em] &= \dfrac{1(-4) + 1(1)}{1 + 1} \\[0.6em] &= \dfrac{-4 + 1}{2} \\[0.6em] &= -\dfrac{3}{2} \end{aligned}

Thus, the coordinates of the point of division are (32,0)\left(-\dfrac{3}{2}, 0\right).

Answer

Ratio is 1:11 : 1 and the coordinates of the point of division are (32,0)\left(-\dfrac{3}{2}, 0\right)

Common Mistakes
  • Axis Coordinate Confusion: Assuming x=0x = 0 instead of y=0y = 0 for a point on the xx-axis. Remember that on the xx-axis, y=0y = 0, while on the yy-axis, x=0x = 0.
  • Coordinate Order: Swapping (x1,y1)(x_1, y_1) with (x2,y2)(x_2, y_2) when multiplying by kk and 11. For ratio k:1k : 1 from AA to BB, kk multiplies the coordinates of BB and 11 multiplies the coordinates of AA.

More questions in Exercise 7.2

Q1

Find the coordinates of the point which divides the join of (1,7)(-1, 7) and (4,3)(4, -3) in the ratio 2:32 : 3.

Q2

Find the coordinates of the points of trisection of the line segment joining (4,1)(4, -1) and (2,3)(-2, -3).

Q3

To conduct Sports Day activities, in your rectangular shaped school ground ABCDABCD, lines have been drawn with chalk powder at a distance of 1 m1\text{ m} each. 100100 flower pots have been placed at a distance of 1 m1\text{ m} from each other along ADAD, as shown in Fig. 7.12. Niharika runs 14th\dfrac{1}{4}\text{th} the distance ADAD on the 2nd line and posts a green flag. Preet runs 15th\dfrac{1}{5}\text{th} the distance ADAD on the eighth line and posts a red flag. What is the distance between both the flags? If Rashmi has to post a blue flag exactly halfway between the line segment joining the two flags, where should she post her flag?

Q4

Find the ratio in which the line segment joining the points (3,10)(-3, 10) and (6,8)(6, -8) is divided by (1,6)(-1, 6).

Q5

Find the ratio in which the line segment joining A(1,5)A(1, -5) and B(4,5)B(-4, 5) is divided by the xx-axis. Also find the coordinates of the point of division.

Q6

If (1,2)(1, 2), (4,y)(4, y), (x,6)(x, 6) and (3,5)(3, 5) are the vertices of a parallelogram taken in order, find xx and yy.

Q7

Find the coordinates of a point A\text{A}, where AB\text{AB} is the diameter of a circle whose centre is (2,3)(2, -3) and B\text{B} is (1,4)(1, 4).

Q8

If A and B are (2,2)(-2, -2) and (2,4)(2, -4), respectively, find the coordinates of P such that AP=37AB\text{AP} = \dfrac{3}{7} \text{AB} and P lies on the line segment AB.

Q9

Find the coordinates of the points which divide the line segment joining A(2,2)A(-2, 2) and B(2,8)B(2, 8) into four equal parts.

Q10

Find the area of a rhombus if its vertices are (3,0)(3, 0), (4,5)(4, 5), (1,4)(-1, 4) and (2,1)(-2, -1) taken in order.

[Hint : Area of a rhombus = 12\dfrac{1}{2} (product of its diagonals)]

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