Polar Form of a Complex Number

Algebra Level 3

The complex number x = 3 i x= \sqrt{3} - i can be expressed in polar form x = r ( cos θ + i sin θ ) x = r\left(\cos\theta + i\sin\theta \right) , where r r is a positive real number and 0 θ 2 π 0 \leq \theta \leq 2\pi . In fact, θ = a π b \theta = \frac{a\pi}{b} where a a and b b are coprime positive integers. What is the value of a + b a+b ?

Details and assumptions

i i is the imaginary unit, where i 2 = 1 i^2=-1 .


The answer is 17.

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7 solutions

DreamRunner Moshi
Dec 31, 2013

x = 3 ı x \space=\space \sqrt{3}-\imath x = 2 ( 3 2 ı 1 2 ) x \space = \space 2(\frac{\sqrt{3}}{2}-\imath\frac{1}{2}) x = 2 ( cos ( 2 π π 6 ) ı sin ( 2 π π 6 ) ) x \space= \space 2(\cos{(2\pi-\frac{\pi}{6})-\imath\sin{(2\pi-\frac{\pi}{6}})}) x = 2 ( cos 11 π 6 ı sin 11 π 6 ) x\space=\space2(\cos{\frac{11\pi}{6}}-\imath\sin{\frac{11\pi}{6}})

Now θ = 11 π 6 \theta \space = \space \frac{11\pi}{6} . So, answer 11+6= 17 \boxed{17}

Muhammad Shariq
Dec 31, 2013

We have that x = a + b i = 3 i a = 3 , b = 1 \large x=a+bi=\sqrt{3}-i \implies a=\sqrt{3},b=-1

Converting the complex number to its polar form gives us:

z = r ( cos ( θ ) + i sin ( θ ) ) = ( 3 ) 2 + ( 1 ) 2 ( cos ( 11 π 6 ) + i sin ( 11 π 6 ) ) \large z=r(\cos(\theta)+i\sin(\theta))=\sqrt{(\sqrt{3})^2+(-1)^2}\left(\cos\left(\frac{11 \pi}{6}\right)+i\sin\left(\frac{11 \pi}{6}\right)\right) = 4 ( cos ( 11 π 6 ) + i sin ( 11 π 6 ) ) \large = 4\left(\cos\left(\frac{11 \pi}{6}\right)+i\sin\left(\frac{11 \pi}{6}\right)\right)

Therefore a + b = 11 + 6 = 17 \large a+b=11+6=\boxed{17} .

Budi Utomo
Dec 31, 2013

r = 2. So 3^(1/2) - i = 2 ( cos P + i sin P ) --> 3^(1/2)/2 - i/2 = cos P + i sin P. because cos P is positive and sin P is negative so the angle in 3th cuadrant arc cos 3^(1/2)/2 = (360 - 30) = 330. So 330 = 11/6 . 180 = 11/6 . phi ----> a + b = 11 + 6 = 17

r =sqrt(sqrt3^2+(-1)^2) = 2 tan theta = 1/sqrt3 =30 since it is in the fourth quadrant so theta = 360-30 = 330 we should put it in the degree form so theta = 330pi/180 = 11pi/6 so a+b =11+6 =17 i think it is easier that way :D

Sherif Elmaghraby - 7 years, 5 months ago
Vishwanath Pai
Jan 6, 2014

first part is positive and second negative so cos is positive and sin is negative so theta ends in 4th quadrant so answer will be 11/6 or a+b = 17

the ratio of cos to sin is - \sqrt{3} which means tan theta is - \sqrt{3}. Which means theta is n*\pi - \frac{\pi}{6} . But since r is positive. It has to be 2\pi - \frac{\pi}{6} since \pi - \frac{\pi}{6} will give negative of what we want.

Utkarsh Sinha
Jan 5, 2014
  • X = √3 – i
  • X = 2 ((√3/2) + i(-1/2))
  • X = 2 (cos(11π/6) + isin(11π/6))
  • Comparing with x = r[cosθ + isinθ]
  • Θ = 11π/6
  • Comparing with Θ = aπ/b; a = 11, b = 6
  • Therefore, a + b = 17.

[I haven't really understand trig. I was also guessing the r r , but I think my solution is right]

From the problems, we have x = r ( cos θ + i sin θ ) = 3 i x=r(\cos\theta+i\sin\theta)=\sqrt{3}-i , hence we also have

3 r cos θ = r i sin θ + i . . . ( 1 ) \sqrt{3}-r\cos\theta=ri\sin\theta+i...(1)

Like I said, I was guessing for the value of r r . And I get the possible value of r r is 2 2 .

The right side of (1) is arbitrary, meanwhile its left side is real number. Therefore, it must follow that the right side is also real number. And the only possible real value of the right side of equation is

2 i sin θ + i = 0... ( 2 ) 2i\sin\theta+i=0...(2)

Simplify this we get sin θ = 1 2 \sin\theta=- \frac{1}{2} . One of the solution to the equation is θ = 33 0 \theta=330^\circ .

Recall that 2 i sin θ + i = 0 2i\sin\theta+i=0 , thus 3 2 cos θ = 0 \sqrt{3}-2\cos\theta=0 . We can use the last equation to verify (2).

cos θ = 3 2 θ = 33 0 \cos\theta=\frac{\sqrt{3}}{2}\Leftrightarrow\theta=330^\circ [this is one of the values of θ \theta ]

Hence, we conclude θ = 33 0 = 11 π 6 \theta=330^\circ=\frac{11\pi}{6} . And finally a + b = 11 + 6 = 17 a+b=11+6=\boxed{17}

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