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Q.Consider two lines L1:2x+y=4L_1 : 2x + y = 4 and L2:(2x−y=2)L_2 : (2x - y = 2).

a) Find the angle between L1L_1 and L2L_2.
(2)
b) Find the equation of the line passing through the intersection of L1L_1 and L2L_2 which makes an angle 45∘45^\circ with the positive direction of xx-axis.
(3)
c) Find the xx and yy intercepts of the third line obtained in the above question (b). (1)
Kerala DhseKerala DHSE Plus One Board 2019Subjective· 6mImportance★★★★★
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Get each line's slope, use the angle-between-lines formula; find the intersection point, then write a line through it with slope tan⁡45∘\tan45^\circ; finally read off the intercepts by setting x=0x=0 and y=0y=0.

a) Angle between L1:2x+y=4L_1: 2x+y=4 and L2:2x−y=2L_2: 2x-y=2

Slopes: L1:y=−2x+4⇒m1=−2L_1: y=-2x+4 \Rightarrow m_1=-2. L2:y=2x−2⇒m2=2L_2: y=2x-2 \Rightarrow m_2=2.

tan⁡θ=∣m1−m21+m1m2∣=∣−2−21+(−2)(2)∣=∣−4−3∣=43\tan\theta = \left|\dfrac{m_1-m_2}{1+m_1m_2}\right| = \left|\dfrac{-2-2}{1+(-2)(2)}\right| = \left|\dfrac{-4}{-3}\right| = \dfrac43

θ=tan⁡−1(43)≈53.13∘\theta = \tan^{-1}\left(\dfrac43\right)\approx53.13^\circ

b) Line through the intersection of L1,L2L_1,L_2 at 45∘45^\circ to the xx-axis

Intersection: solve 2x+y=42x+y=4 and 2x−y=22x-y=2. Adding: 4x=6⇒x=324x=6\Rightarrow x=\dfrac32. Then y=4−2(32)=1y=4-2\left(\dfrac32\right)=1.

Intersection point: (32,1)\left(\dfrac32,1\right).

Required slope =tan⁡45∘=1=\tan45^\circ=1. Using point-slope form:

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