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ICSE Class X Math 2014 Paper

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Question : 8 of 52
Marks: +1, -0
Calculate the ratio in which the line joining A(−4,2)A(-4,2) and B(3,6)B(3,6) is divided by point P(xP(x , 3). Also find (i) xx (ii) Length of APA P .
Solution:  
Let P(x,3)P(x, 3) divide the line segment joining the points A(−4,2)A(-4,2) and B(3,6)B(3,6) in the ratio k:1k: 1
∴\therefore Coordinates of PP is
(m1x2+m2x1m1+m2,  m1y2+m2y1m1+m2)\left( \frac{m_1 x_2+m_2 x_1}{m_1+m_2},\; \frac{m_1 y_2+m_2 y_1}{m_1+m_2} \right) =(3k−4k+1,  6k+2k+1)= \left( \frac{3 k-4}{k+1},\; \frac{6 k+2}{k+1} \right)
But coordinate of PP is (x,3)(x, 3)
⇒    6k+2k+1=3\Rightarrow \;\; \frac{6k+2}{k+1}=3
6k+2=3k+36k+2=3k+3
3k=1⇒k=133k=1 \Rightarrow k=\frac{1}{3}
∴\therefore The required ratio is 13:1\frac{1}{3}: 1 i.e., 1:31: 3 (internally)
(i) Now x=3k−4k+1x=\frac{3k-4}{k+1}
Putting k=13k=\frac{1}{3} , we get
x=3×13−413+1x=\frac{3 \times \frac{1}{3} - 4}{\frac{1}{3} + 1}
=1−41+33=−343=−94= \frac{1-4}{\frac{1+3}{3}} = \frac{-3}{\frac{4}{3}} = \frac{-9}{4}
(ii) ∴\therefore Coordinate of PP is (−94,3)\left( \frac{-9}{4}, 3 \right)
Length of AP=(x2−x1)2+(y2−y1)2\text{Length of AP} = \sqrt{(x_2-x_1)^2 + (y_2-y_1)^2}
=(−94+4)2+(3−2)2.= \sqrt{ \left( -\frac{9}{4} + 4 \right)^2 + (3-2)^2 }.
=(−9+164)2+(1)2= \sqrt{ \left( \frac{-9+16}{4} \right)^2 + (1)^2 }
=4916+1=49+1616= \sqrt{ \frac{49}{16} + 1 } = \sqrt{ \frac{49+16}{16} }
=6516=654= \sqrt{ \frac{65}{16} } = \frac{ \sqrt{65} }{4}
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