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Find the modulus and amplitude of the following complex numbers. 7 − 5i

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Question

Find the modulus and amplitude of the following complex numbers.

7 − 5i

Sum
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Solution

Let z = 7 − 5i

Here a = 7, b = − 5 i.e., a < 0, b > 0

∴ |z| = `sqrt(a^2 + b^2)`

= `sqrt(7^2 + (-5)^2`

= `sqrt(49 + 25)`

= `sqrt(74)`

Here, (7, -5) lies in 4th quadrant.

amp (z) = θ = `tan^-1("b"/"a")+2pi`

θ = `tan^-1((-5)/7)+2pi`

θ = `-tan^-1(5/7) + 2pi`   ...[∵ tan(– θ) = – tanθ]

Hence, modulus = `sqrt(74)` and  amplitude = `-tan^-1(5/7) + 2pi`

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Chapter 1: Complex Numbers - Exercise 1.3 [Page 15]

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