1.

Find the condition for two straight lines to be perpendicular

Answer»

let slope of lines l1 and l2 are respectively m1 and m2.

we know that the the angle between two lines whose slopes are m1 & m2 is given by

tanθ = \(\frac{m_1\,-\,m_2}{1\,+\,m_1\,m_2},\).....(1) 

where θ is angle between lines l1 and l2.

for lines l1 & l2 to be perpendicular,

we have θ = 90°

\(\Rightarrow\) tanθ = tan90° = ∞ = \(\frac{1}{0}\)

therefore, equation (i), we have 

\(\frac{m_1\,-\,m_2}{1\,+\,m_1\,m_2}=\frac{1}{0}\)

\(\Rightarrow\) 1 + m1 m2 = 0

\(\Rightarrow\) m1 m2 = -1.

Hence, two straight line are perpendiculer is the product of there slopes is equal to -1.



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