11th Grade > Mathematics
COMPLEX NUMBERS MCQs
:
C
Let z = i[1+3+5+......+(2n+1)]
Clearly series is A.P with common difference = 2
∵ Tn = 2n-1 And Tn+1 = 2n+1
So, number of terms in A.P. = n+1
Now, sn+1=n+12 [2.1+(n+1-1)2]
⇒ Sn+1=n+12[2+2n]=(n+1)2 i.e., i(n+1)2
Now put n = 1,2,3,4,5,........
n = 1, z = i4 = 1, n = 2, z = i5 = -1 ,
n = 3, z = i8 = 1, n = 4, z = i10 = -1 ,
n = 5, z = i12 = 1, .........
:
A
Let z = x + iy, ¯z = x -iy and z−1=1x+iy
⇒ ¯¯¯¯¯¯¯¯z−1=x+iyx2+y2 ; ∴ ¯¯¯¯¯¯¯¯z−1¯z=x+iyx2+y2(x−iy) = 1
:
D
sinx+icos2x and cosx-isin2xare conjugate to each other if sinx=cosx and cos2x=sin2x
Or tan x = 1 ⇒ x = π4,5π4,9π4, .......... ...............(i)
And tan 2x = 1 ⇒ 2x = π4,5π4,9π4, ..........
or x = π8,5π8,9π8, .......... ...............(ii)
There exists no value of x common in (i) and (ii). Therefore there is no value of x for which the given complex numbers are conjugate.
:
C
let z=r (cosθ+isinθ)
Then ∣∣z+1z∣∣=a ⇒ ∣∣z+1z∣∣2=a2
⇒ r2+1r2+2cosθ = a2 ⋯ ⋯(i)
Differentiating w.r.t θ we get
2rdrdθ-2r3drdθ-4sin2θ
Putting drdθ=0, we get θ=0,π2
r is maximum for θ = 0, π2, therefore from (i)
r2+1r2−2=a2⇒r−1r=a⇒r=a+2√a2+42
:
B
√3+i=(a+ib)(c+id)
∴ac−bd=√3 and ad+bc=1
Now tan−1(ba)+tan−1(dc)
= tan−1(ab+dc1−ba.dc)= tan−1(bc+adac−bd)= tan−1(1√3)
=nπ+π6, nϵI
:
A
Since1-i = √2 [cosπ4−isinπ4], |1-i|
∴ |1−i|x=2x ⇒ (√2)x=2x ⇒ 2x/2 =2x
⇒ x2=x then x=0.
Therefore, the number of non-zero integral solutions is nil or Zero.
:
B
Let √3−4i=x+iy ⇒ 3-4i= x2-y2+2ixy
⇒ x2-y2=3, 2xy=-4 ..........(i)
⇒ (x2+y2)2=(x2−y2)2+4x2y2=(3)2+(−4)2=25
⇒ (x2+y2)2=5 ............(ii)
From equation (i) and (ii) ⇒ x2=4 ⇒x=±2,
y2=1 ⇒y=±1.Hence the square root of (3-4i)
is ±(2-i).
:
A
c+ic−i = a+ib .........(i)
∴ c−ic+i = a-ib .........(ii)
Multiplying (i) and (ii), we get
c2+1c2+1=a2+b2⇒a2+b2=1.
:
C
z = (2+i)23+i=3+4i3+i×3−i3−i=1310+i910
Conjugate = 1310−i910.
:
B
we have
(1+i)(1+2i)(1+3i)......(1+ni) = a+ib .......(i)
⇒ (1+i)(1+2i)(1+3i)......(1+ni) = a-ib ........(ii)
Multiplying (i) and (ii), we get 2.5.10.....(1+n2) = a2+b2