To answer this question, we define the angles a, b and c from the picture below:
From the picture we see the following:
1) The lines t and l_1 are perpendicular because the angle between them is equal to 90°.
2) The angles 120° and b are supplementary, so they sum up 180°. Taking to account this, we have:
[tex]\begin{gathered} 120^{\circ}+b=180^{\circ}, \\ b=180^{\circ}-120^{\circ}=60^{\circ}\text{.} \end{gathered}[/tex]
3) The angles c and 60° are supplementary, so they sum up 180°. Taking to account this, we have:
[tex]\begin{gathered} c+60^{\circ}=180^{\circ}, \\ b=180^{\circ}-60^{\circ}=120^{\circ}\text{.} \end{gathered}[/tex]
From points 2 and 3 we conclude that lines l_2 and l_3 are parallel.
Answer
a.
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