Q16 / 108 · NTA Abhyas (JEE Main) 2020 · Mathematics · Complex Number

Let Z=reiθ ( r>0 and π<θ≤3π ) is a root of the equation Z8-Z7+Z6-Z5+Z4-Z3+Z2-Z+1=0. If the sum of all values of θ is kπ, then k is equal to

Correct answer: 16

Solution

Given equation is -Z9-1-Z-1=0 
⇒Z9=-1 where Z≠-1
⇒Z=ei2kπ9, where k=5,6,7,8,10,11,12,13
⇒ θ=10π9,12π9,14π9,16π9,20π9,22π9,24π9,26π9
⇒ the sum of all values of θ is equal to 16π
⇒ k=16

Hence, the correct answer is 16.

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