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If z1 and z2 both satisfy z+z¯=2|z-1| arg(z1-z2)=π4, then find ImIm(z1+z2).

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Question

If z1 and z2 both satisfy `z + barz = 2|z - 1|` arg`(z_1 - z_2) = pi/4`, then find `"Im" (z_1 + z_2)`.

Sum
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Solution

Let z = x + iy, z1 = x1 + iy1 and z2 = x2 + iy2 .

Then `z + barz = 2|z - 1|`

⇒ (x + iy) + (x – iy) = `2|x - 1 + "i"y|`

⇒ 2x = 1 + y2    .......(1)

Since z1 and z2 both satisfy (1), we have

`2x_1 = 1 + y_1^2 .....` and `2x_2 = 1 + y_2^2`

⇒ `2(x_1 - x_2) = (y_1 + y_2)(y_1 - y_2)`

⇒ 2 = `(y_1 + y_2) ((y_1 - y_2)/(x_1 - x_2))`  ......(2)

Again `z_1 - "z"_2 = (x_1 - x_2) + "i"(y_"i" - y_2)`

Therefore, tanθ = `(y_1 - y_2)/(x_1 - x_2)`, where θ = arg`("z"_1 - "z"_2)`

⇒ `tan  pi/4 = (y_1 - y_2)/(x_1 - x_2)`  ......`("Since"  theta = pi/4)`

i.e., 1 = `(y_1 - y_2)/(x_1 - x_2)`

From (2), We get 2 = y1 + y2 i.e., `"Im" ("z"_1 + "z"_2)` = 2

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Chapter 5: Complex Numbers and Quadratic Equations - Solved Examples [Page 83]

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NCERT Exemplar Mathematics [English] Class 11
Chapter 5 Complex Numbers and Quadratic Equations
Solved Examples | Q 15 | Page 83

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