CBSE 11th Standard Maths Subject Straight Lines Ncert Exemplar 2 Marks Questions With Solution 2021
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CBSE 11th Standard Maths Subject Straight Lines Ncert Exemplar 2 Marks Questions With Solution 2021
11th Standard CBSE
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Reg.No. :
Mathematics
2 Marks
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A variable line passes through a fixed point P. The algebraic sum of the perpendiculars drawn from the points (2, 0), (0, 2) and (1, 1) on the line is zero. Find the coordinates of the point P.
(a) -
If p is the length of perpendicular from the origin on the line \(\frac { x }{ a } +\frac { y }{ b } =1\) and a2 , p2 and b2 are in AP, then show that a4 + b4 = 0.
(a) -
If the coordinates of the middle point of the portion of a line intercepted between the coordinate axes is (3, 2), then find the equation of the line.
(a) -
Find the equation of line passing through the point \((a\ { cos }^{ 3 }\theta ,\ a \ sin^{ 3 } \ \theta )\) and perpendicular to the line \(x \sec \theta+y \operatorname{cosec} \theta=a\) .
(a) -
A ray of light passing through the point (1, 2) reflects on the x-axis at point A and the reflected ray passes through the point (5, 3). Find the coordinates of A.
(a)
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CBSE 11th Standard Maths Subject Straight Lines Ncert Exemplar 2 Marks Questions With Solution 2021 Answer Keys
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Let slope of the line be m and the coordinates of fixed point P be (x1, y1).
Then, equation of line is y - y1 = m (x - x1) ....(i)
Let the given points be A(2, 0), B(0, 2) and C(1,1).
Now, perpendicular distance from A
\(=\frac { 0-{ y }_{ 1 }-m\left( 2-{ x }_{ 1 } \right) }{ \sqrt { 1+{ m }^{ 2 } } } \)
Perpendicular distance from B \(=\frac { 2-{ y }_{ 1 }-m\left( 0-{ x }_{ 1 } \right) }{ \sqrt { 1+{ m }^{ 2 } } } \)
and perpendicular distance from C \(=\frac { 1-{ y }_{ 1 }-m\left( 1-{ x }_{ 1 } \right) }{ \sqrt { 1+{ m }^{ 2 } } } \)
Now,\(\frac { \left[ -{ y }_{ 1 }-2m+{ mx }_{ 1 }+2-{ y }_{ 1 }+{ mx }_{ 1 }+1-{ y }_{ 1 }-m+{ mx }_{ 1 } \right] }{ \sqrt { 1+{ m }^{ 2 } } } =0\)
\(\Rightarrow \) -3y1 - 3m +3mx1 + 3 = 0
\(\Rightarrow \) -y1 - m + mx1 + 1 = 0
\(\Rightarrow \) y1 = -m + mx1 + 1
On substituting this value in Eq. (i), we gwt
y + m - mx1 - 1 = mx - mx1 \(\Rightarrow \) y - 1 = m(x - 1)
Thus,(x1, y1) = (1, 1) -
Given equation of line is
\(\frac { x }{ a } +\frac { y }{ b } =1\) ....(i)
Perpendicular length from the origin to the line (i) is
\(p=\frac { 1 }{ \sqrt { \frac { 1 }{ { a }^{ 2 } } +\frac { 1 }{ { b }^{ 2 } } } } =\frac { ab }{ \sqrt { { a }^{ 2 }+{ b }^{ 2 } } } \Rightarrow { p }^{ 2 }=\frac { { a }^{ 2 }{ b }^{ 2 } }{ { a }^{ 2 }+{ b }^{ 2 } } \)
Since, a2 , p2 and b2 are in AP.
\(\therefore { 2p }^{ 2 }={ a }^{ 2 }+{ b }^{ 2 }\Rightarrow \frac { 2{ a }^{ 2 }{ b }^{ 2 } }{ { a }^{ 2 }+{ b }^{ 2 } } ={ a }^{ 2 }{ +b }^{ 2 }\)
\(\Rightarrow \) 2a2b2 = (a2 + b2)2 \(\Rightarrow \)2a2b2 = a4+b4+2a2b2
\(\Rightarrow \) a4 + b4 = 0 -
Let equation of line
Since, the coordinates of the middle point are P(3, 2)
\(\therefore \) \(3=\frac { 0+a }{ 3 } \Rightarrow 3=\frac { a }{ 2 } \Rightarrow a=6\)
Similarly, b = 4
Ans. 2x + 3y = 12 -
Given point is \((a \ { cos }^{ 3 }\theta ,a sin^{ 3 }\ \theta )\) and the line is \(x \sec \theta+y \operatorname{cosec} \theta=a\)
\(\because\) Slope of given line \(=-\frac { sec\theta }{ cosec\theta } =-tan\theta \)
\(\therefore\) Slope of required line = \(\frac { 1 }{ tan\theta } =cot\theta \)
Now, equation of the required line having slope cot \(\theta\) and passing through \((a \ { cos }^{ 3 }\theta ,a sin^{ 3 }\ \theta )\), is
\(y-a \sin ^{3} \theta=\cot \theta\left(x-a \cos ^{3} \theta\right) \)
\(\Rightarrow y-a \sin ^{3} \theta=\frac{\cos \theta}{\sin \theta}\left(x-a \cos ^{3} \theta\right) \)
\(\Rightarrow y \sin \theta-a \sin ^{4} \theta=x \cos \theta-a \cos ^{4} \theta \)
\(\Rightarrow x \cos \theta-y \sin \theta=a \cos ^{4} \theta-a \sin ^{4} \theta \)
\(\Rightarrow x \cos \theta-y \sin \theta \)
\(=a\left[\left(\cos ^{2} \theta+\sin ^{2} \theta\right)\left(\cos ^{2} \theta-\sin ^{2} \theta\right)\right] \)
\(\Rightarrow x \cos \theta-y \sin \theta=a \cos 2 \theta\) -
Let the coordinates of the point A be ( x, 0 ) .From the figure, the slope of the reflected ray is given by
tan \(\theta \) = \(\frac{3}{5 - x} \) .... ( i )
Again , the slope of the incident ray is given by
tan ( \(\pi\) - \(\theta\) ) = \(\frac{-2}{ x-1 }\) [ \(\because\) m = \(\frac{y_2-y_1}{x_2-x_1}\) ]
\(\Rightarrow\) - tan \(\theta\) = \( \frac{-2}{ x-1 }\) [ \(\because\) tan ( \(\pi\) - \(\theta\) = - tan \(\theta\) ) ]
\(\Rightarrow\) tan \(\theta\) = \( \frac{2}{ x - 1}\)
From Eqs.(i) and (ii), we get
\(\frac{3}{ 5 - x }\) = \(\frac{2}{ x - 1}\)
\(\Rightarrow\) 3x - 3 = 10 -2x
\(\Rightarrow\) x = \( \frac{13}{5}\)
Therefore , the required coordinates of the point A are ( \(\frac{13}{5}\),0 ).