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à 5.2ïTrigonometric Form of Complex Numbers
äïPlease change the following complex numbers from rectan-
êêgular form to trigonometric form.
âS
êêè 1)ï1 + iï=ï√2∙(cos 45° + i∙sin 45°)
êêè 2)ï-√3 + iï=ï2∙(cos 150° + i∙sin 150°)
êêè 3)ï1 - √3∙iï=ï2∙(cos 300° + i∙sin 300°)
#éSïThe trigonometric form of a complex number, x¬ + y¬i, is de-
#êêêêêêè ┌─────────────
#fined to be r∙(cos Θ + i∙sin Θ), whereïrï=ïá(x¬)ì + (y¬)ì,èx¬ =
#r∙cos Θ, and y¬ = r∙sin Θ with 0 ≤ Θ ≤ 360°.
#êêêè In the figure, the complex number, x¬ + y¬i,
êêê is a point in the complex number plane, and it
êêê is graphed in the same way that you would graph
#êêê the ordered pair, (x¬, y¬).ïThe right triangle
êêê suggests using the Pythagorean Theorem, so it
êêê is natural to define r to be the square root of
#êêê x¬ì plus y¬ì.ïIt is also natural to use right
@fig5201.bmp,10,145
#êêê triangle trigonometry to find expressions for x¬
#êêê and y¬.ïCos Θ = x¬/r and sin Θ = y¬/r.ïThus,
#êêê x¬ = r∙cos Θ and y¬ = r∙sin Θ.ïTherefore, by
#êêê substitution x¬ + y¬iï=ïr∙cos Θ + r∙sin Θ ∙i =
êêê r∙(cos Θ + i∙sin Θ).
êêêëIn the example, 1 + i can be expressed in
êêê trigonometric form by finding r and Θ.
#êêêë┌──────────è┌────────è┌──
#êêê r = á x¬ì + y¬ì = á 1ì + 1ì = á 2
#êêê The reference angle is Θ¬ = Tanúî y¬/x¬ =
#êêê Tanúî(1/1) = 45°.ïSince the complex number,1 + i,
êêê is in the first quadrant, the angle is Θ = 45°.
êêêëAlso in the example, the complex number,
êêê -√3 + i, can be expressed in trigonometric form
êêê by finding r and Θ.
#êêêë┌────────────è┌──────
#êêê r = á (-√3)ì + 1ì = á 3 + 1 = 2
#The reference angle Θ¬ = Tanúî(-1/√3) = -30°.ïSince the complex number,
-√3 + i, is in the second quadrant, the angle Θ is 150°.ïThus, -√3 + i
= 2∙(cos 150° + i∙sin 150°).
è The last complex number in the example is 1 - √3∙i.ïFor this com-
#êêè┌───────────
#plex number, r = á 1ì + (√3)ì = 2, and the reference angle Θ¬ =
#Tanúî(-√3/1) = -60°.ïSince the complex number, 1 - √3∙i, is in the
fourth quadrant, Θ is 300°.ïThus, 1 - √3∙i = 2∙(cos 300° + i∙sin 300°).
1ë Find trigonometric form for the complex number
êêêêè -1 + i
êêêA)ï2∙(cos 45° + i∙sin 45°)
êêêB)ï√2∙(cos -45° + i∙sin -45°)
êêêC)ï√2∙(cos 135° + i∙sin 135°)
êêêD)ïå of ç
üê Find trigonometric form for the complex number
êêêêè -1 + i
êë ModulusêêêèReference Angle
#êè┌───────────
#ë r = á (-1)ì + 1ì = √2êëΘ¬ = Tanúî(-1/1) = -45°
Since -1 + i is in the second quadrant, Θ = 135°.ïThus, -1 + i =
√2∙(cos 135° + i∙sin 135°).
Ç C
2ë Find trigonometric form for the complex number
êêêêè -1 - i
êêêA)ï2∙(cos 45° + i∙sin 45°)
êêêB)ï√2∙(cos 225° + i∙sin 225°)
êêêC)ï√2∙(cos 45° + i∙sin 45°)
êêêD)ïå of ç
üê Find trigonometric form for the complex number
êêêêè -1 - i
êë ModulusêêêèReference Angle
#êè┌──────────────
#ë r = á (-1)ì + (-1)ì = √2êëΘ¬ = Tanúî(-1/-1) = 45°
Since -1 - i is in the third quadrant, Θ = 225°.ïThus, -1 - i =
√2∙(cos 225° + i∙sin 225°).
Ç B
3ë Find trigonometric form for the complex number
êêêêè -1 + √3∙i
êêêA)ï2∙(cos 120° + i∙sin 120°)
êêêB)ï√2∙(cos 60° + i∙sin 60°)
êêêC)ï2∙(cos -60° + i∙sin -60°)
êêêD)ïå of ç
üê Find trigonometric form for the complex number
êêêêè -1 + √3∙i
êë ModulusêêêèReference Angle
#êè┌──────────────
#ë r = á (-1)ì + (√3)ì = √4 = 2ê Θ¬ = Tanúî(√3/-1) = -60°
Since -1 + √3∙i is in the second quadrant, Θ = 120°.ïThus, -1 + √3∙i =
2∙(cos 120° + i∙sin 120°).
Ç A
4ë Find trigonometric form for the complex number
êêêêè √3 + i
êêêA)ï2∙(cos 150° + i∙sin 150°)
êêêB)ï2∙(cos 30° + i∙sin 30°)
êêêC)ï√2∙(cos 150° + i∙sin 150°)
êêêD)ïå of ç
üê Find trigonometric form for the complex number
êêêêè √3 + i
êë ModulusêêêèReference Angle
#êè┌──────────────
#ë r = á (√3)ì + (1)ì = √4 = 2ê Θ¬ = Tanúî(1/√3) = 30°
Since √3 + i is in the first quadrant, Θ = 30°.ïThus, √3 + i =
2∙(cos 30° + i∙sin 30°).
Ç B
5ë Find trigonometric form in radian measure for the complex
êê number √3 - i.
êêêA)ï2∙(cos 11π/6 + i∙sin 11π/6)
êêêB)ï2∙(cos π/6 + i∙sin π/6)
êêêC)ï√2∙(cos π/3 + i∙sin π/3)
êêêD)ïå of ç
üê Find trigonometric form for the complex number
êêêêè √3 - i
êë ModulusêêêèReference Angle
#êè┌──────────────
#ë r = á (√3)ì + (-1)ì = √4 = 2ê Θ¬ = Tanúî(-1/√3) = -π/6
Since √3 - i is in the fourth quadrant, Θ = 11π/6.ïThus, √3 - i =
2∙(cos 11π/6 + i∙sin 11π/6).
Ç A
6ë Find trigonometric form for the complex number
êêêêê2i
êêêA)ï2∙(cos 180° + i∙sin 180°)
êêêB)ï2∙(cos 270° + i∙sin 270°)
êêêC)ï2∙(cos 90° + i∙sin 90°)
êêêD)ïå of ç
üê Find trigonometric form for the complex number
êêêêë2i
êë ModulusêêêèReference Angle
#êè┌──────────────êêïTanúî(2/0) is undefined, but
#ë r = á (0)ì + (2)ì = √4 = 2êïby inspection Θ = 90°.
Thus, 2i = 2∙(cos 90° + sin 90°).
Ç C
7ë Find trigonometric form in radian measure for the complex
êê number 3 + 7i.
êêêA)ï3.241∙(cos 2.718 + i∙sin 2.718)
êêêB)ï7.616∙(cos 1.166 + i∙sin 1.166)
êêêC)ï4.165∙(cos 4.281 + i∙sin 4.281)
êêêD)ïå of ç
üê Find trigonometric form for the complex number 3 + 7i.
You will need to use your calculator to find r and Θ.
êë ModulusêêêèReference Angle
#êè┌────────────è┌──
#ë r = á (3)ì + (7)ì = á58 ≈ 7.616è Θ¬ = Tanúî(7/3) ≈ 1.166
Since 3 + 7i is in the first quadrant, Θ ≈ 1.616.ïThus, 3 + 7i ≈
7.616∙(cos 1.166 + i∙sin 1,166).
Ç B
äïPlease change the following complex numbers from trigono-
êêmetric form to rectangular form.
â
ê4∙(cos 150° + i∙sin 150°) = 4∙(-√3/2 + i∙(1/2)) = -2∙√3 + 2i
éSïTo change a complex number from trigonometric form to rectangu-
lar form, you should find the cos and sin of the given angle and distri-
bute the given value of r.
ê4∙(cos 150° + i∙sin 150°) = 4∙(-√3/2 + i∙(1/2)) = -2∙√3 + 2i
8è Change the given complex number to rectangular form.
êêêï√2∙(cos 315° + i∙sin 315°)
êè A)ï-1 + iêêêèB)ï2 - 2i
êè C)ï1 - iêêêè D)ïå of ç
ü
êêêï√2∙(cos 315° + sin 315°)
êêêï=ï√2∙(1/√2 + i(-1/√2))
êêêï=ï1 - i
Ç C
9è Change the given complex number to rectangular form.
êêêï2∙(cos 240° + i∙sin 240°)
êè A)ï-1 - √3iêêê B)ï1 - √3i
êè C)ï-1 + √3iêêê D)ïå of ç
ü
êêêï2∙(cos 240° + sin 240°)
êêêï=ï2∙(-1/2 + i(-√3/2))
êêêï=ï-1 - √3i
Ç A
10èChange the given complex number to rectangular form.
êêêï2∙(cos 4π/3 + i∙sin 4π/3)
êè A)ï-1 + √3iêêê B)ï-1 - √3i
êè C)ï1 - √3iêêêïD)ïå of ç
ü
êêêï2∙(cos 4π/3 + sin 4π/3)
êêêï=ï2∙(-1/2 + i(-√3/2))
êêêï=ï-1 - √3i
Ç B
11èChange the given complex number to rectangular form.
êêêï2.4∙(cos 23° + i∙sin 23°)
êè A)ï2.2092 + .8897iêê B)ï2.163 - .8861i
êè C)ï3.614 + .2183iêêïD)ïå of ç
ü
êêêï2.4∙(cos 23° + sin 23°)
êêêï≈ï2.4∙(.9205 + i.3707)
êêêï≈ï2.2092 + .8897i
Ç A
12èChange the given complex number to rectangular form.
êêêï3∙(cos 270° + i∙sin 270°)
êè A)ï-1 - 3iêêêïB)ï1 + 3i
êè C)ï-3iêêêë D)ïå of ç
ü
êêêï3∙(cos 270° + i∙sin 270°)
êêêï=ï3∙(0 + i∙(-1))
êêêï=ï-3i
Ç C