# 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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