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Q.

Value(s) of (-i)13 is/are:


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a

3-i2

b

3+i2

c

-3+i2

d

None 

answer is A.

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Detailed Solution

We can write (-i)13=z.
Cubing both the sides, we will get:
 −i=z3
Using the value −i=i3 and bringing all the terms to the same side of the equation, we get:
z3-i3
  On using the factorization a3-b3=(a-b)(a2+ab+b2) , we have:
 (z−i)( z2 +zi+i2)=0
Using i2=−1 and the fact that the product of two terms can be 0 only if at least one of them is 0, we conclude the following:
 z−i=0 OR z2+iz−1=0
Using the quadratic formula, we get:
 z=i OR z=-i±i2-4(1)(-1)2(1)
 z=i OR z= -i±-1+42
 z=i OR z= = -i±32 OR
  z= = -i-32
∴ The correct answer options are  3-i2 and -3-i2.
 
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