If x + 4y – 5 = 0 and 4x + ky + 7 = 0 are two perpendicular lines then k is

Solution :

\(m_1\) = -\(1\over 4\)  \(m_2\) = -\(4\over k\)

Two lines are perpendicular if \(m_1 m_2\) = -1

\(\implies\)  (-\(1\over 4\))\(\times\)(-\(4\over k\)) = -1  \(\implies\)  k = -1

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