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Find the modulus and amplitude of the following complex numbers. −8 + 15i - Mathematics and Statistics

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Question

Find the modulus and amplitude of the following complex numbers.

−8 + 15i

Sum
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Solution

Let z = −8 + 15i

Here, a = −8 , b = 15, i.e., a < 0, b > 0
∴ |z| = `sqrt("a"^2 + "b"^2)`

= `sqrt((-8)^2 + 15^2`

= `sqrt(64 + 225)`

= `sqrt(289)`

= 17

Here, (-8, 15) lies in 2nd quadrant.

∴ amp (z) = ` tan^-1("b"/"a") + pi`

= `tan^-1(15/(-8)) + pi`

= `-tan^-1(15/8) + pi`   ...[∵ tan–1(– θ) = – tan–1θ]

Hence, modulus = 17 and amplitude = `-tan^-1(15/8) + pi`.

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

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