1.

What is the angle between the lines whose equations are: 3x + y – 7 = 0 and x + 2y + 9 = 0.(a) 45º (b) tan–1 \(\big(\frac{5}{2}\big)\)(c) 135º (d) tan–1 \(\big(\frac{3}{4}\big)\)

Answer»

(a) 45º 

3x + y – 7 = 0 ⇒ y = –3x + 7 ⇒ Slope (m1) = –3 

x + 2y + 9 = 0 ⇒ y = \(\frac{-x}{2}\) - \(\frac{9}{2}\) ⇒ Slope (m2) = \(-\frac{1}{2}\)

If θ is the angle between the given lines, then 

tan θ = \(\big|\frac{m_1-m_2}{1+m_1m_2}\big|\) = \(\bigg|\frac{-3-\big(\frac{1}{2}\big)}{1+(-3)\big(-\frac{1}{2}\big)}\bigg|\) = \(\bigg|\frac{-\frac{5}{2}}{1+\frac{3}{2}}\bigg|\)

\(\bigg|\frac{-\frac{5}{2}}{\frac{5}{2}}\bigg|\) = 1

∴ θ = 45°.



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