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Q.Find the image of the point (3,8)(3, 8) with respect to the line x+3y=7x + 3y = 7 assuming the line to be a plane mirror. OR Find the distance of the line 4x−y=04x - y = 0 from the point P(4,1)P(4, 1) measured along the line making an angle of 135°135° with the positive xx-axis.

Manipur CohsemCouncil of Higher Secondary Education, Manipur (Higher Secondary 1st Year) 2022Subjective· 4mImportance★★★★★
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Using the standard reflection formula for the line x+3y−7=0x+3y-7=0, the image of (3,8)(3,8) works out to (−1,−4)(-1,-4).

For a line ax+by+c=0ax+by+c=0, the image (reflection) of a point (x1,y1)(x_1,y_1) is given by:

x′=x1−2a(ax1+by1+c)a2+b2,y′=y1−2b(ax1+by1+c)a2+b2x' = x_1 - \dfrac{2a(ax_1+by_1+c)}{a^2+b^2}, \qquad y' = y_1 - \dfrac{2b(ax_1+by_1+c)}{a^2+b^2}

Here the line is x+3y−7=0x+3y-7=0, so a=1, b=3, c=−7a=1,\ b=3,\ c=-7, and (x1,y1)=(3,8)(x_1,y_1)=(3,8).

Compute ax1+by1+c=1(3)+3(8)−7=3+24−7=20ax_1+by_1+c = 1(3)+3(8)-7 = 3+24-7=20, and a2+b2=1+9=10a^2+b^2=1+9=10.

x′=3−2(1)(20)10=3−4=−1x' = 3 - \dfrac{2(1)(20)}{10} = 3-4 = -1

y′=8−2(3)(20)10=8−12=−4y' = 8 - \dfrac{2(3)(20)}{10} = 8-12 = -4

So the image is (−1,−4)(-1,-4).

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