Complex Numbers - Argand Plane

Algebra Level 2

Suppose that we have two complex numbers z 1 = 3 2 i z_1 = 3 - 2i and z 2 = 4 3 i z_2 = 4 - 3i .

In what quadrant is this complex number z 1 z 2 z_1z_2 located?

Quadrant I Quadrant II Quadrant III Quadrant IV

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2 solutions

Mahdi Raza
Aug 9, 2020

The result as product of two complex numbers given is

z 3 = ( 3 2 i ) ( 4 3 i ) = 12 17 i + 6 i 2 = 6 17 i z_{3} = (3-2i)(4-3i) = 12 -17i + 6i^2 = 6-17i

The new complex number we get is z 3 = 6 17 i z_{3} = 6-17i

R e ( 6 ) , I m ( 17 ) \mathfrak{Re}(6), \quad \mathfrak{Im}(-17)

Hence, in the Argand plane, it will be in the IVth quadrant

Brilliant Mathematics Staff
Aug 1, 2020

If we take the product

z 1 z 2 = ( 3 2 i ) ( 4 3 i ) = ( 3 ) ( 4 ) 3 ( 3 i ) 4 ( 2 i ) + ( 2 i ) ( 3 i ) = 12 9 i 8 i + 6 i 2 = 12 17 i + 6 ( 1 ) = 12 17 i 6 = 6 17 i . \begin{array} {r c l } z_1 z_2 &=& (3-2i)(4-3i) \\ &=& (3)(4) - 3(3i) - 4(2i) + (2i)(3i) \\ &=& 12 - 9i - 8i +6i^2 \\ &=& 12 - 17i + 6(-1) \\ &=& 12 - 17i - 6 \\ &=& 6 - 17i. \end{array}

In the Argand plane, this is the equivalent of the point ( 6 , 17 ) (6, -17) , which is in quadrant IV.

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