复数幂公式
复数是具有公式 a + ib 的数,其中 a 和 b 是实数,I (iota) 是虚数部分,表示 (-1),通常以矩形或标准形式表示。例如,10 + 5i 是一个复数,其中 10 代表实部,5i 代表虚部。根据 a 和 b 的值,它们可能是完全真实的或纯虚构的。当a + ib 中a = 0 时,ib 是一个全虚数,而当b = 0 时,我们得到a,它是一个严格实数。
复数幂公式
要根据指定的指数展开复数,必须首先将其转换为其极坐标形式,该形式具有模数和自变量作为分量。之后,应用 DeMoivre 定理,该定理指出:
De Moivre 公式指出,对于一个数字的所有实数值,比如 x,
(cos x + isinx)n = cos(nx) + isin(nx), where n is any integer.
公式的推导
DeMoivre 定理可以在数学归纳法的帮助下导出/证明如下:
As stated previously, (cos x + isinx)n = cos(nx) + isin(nx) …. (1)
For n = 1, we obtain:
(cos x + i sin x)1 = cos(1x) + i sin(1x) = cos(x) + i sin(x), which is true.
Assuming that the formula holds true for any integer, say n = k. Then,
(cos x + i sin x)k = cos(kx) + i sin(kx) …. (2)
Now, we just have to prove that the formula holds true for n = k + 1.
(cos x + i sin x)k+1 = (cos x + i sin x)k (cos x + i sin x)
= (cos (kx) + i sin (kx)) (cos x + i sin x) [Using (i)]
= cos (kx) cos x − sin(kx) sinx + i (sin(kx) cosx + cos(kx) sinx)
= cos {(k+1)x} + i sin {(k+1)x}
⇒ (cos x + i sin x)k+1 = cos {(k+1)x} + i sin {(k+1)x}
Hence the result is proved.
示例问题
问题 1. 展开 (1 + i) 5 。
解决方案:
Here, r = = , θ = π/4
The polar form of (1 + i) =
According to De Moivre’s Theorem: (cosθ + sinθ)n = cos(nθ) + i sin(nθ).
Thus, (1 + i)5 =
=
= −4 − 4i
问题 2:展开 (2 + 2i) 6 。
解决方案:
Here, r = , θ = π/4
The polar form of (2 + 2i) =
According to De Moivre’s Theorem: (cosθ + sinθ)n = cos(nθ) + isin(nθ).
Thus, (2 + 2i)6 =
=
= 512 (-i)
= −512i
问题 3:展开 (1 + i) 18 。
解决方案:
Here, r = , θ = π/4
The polar form of (1+i) =
According to De Moivre’s Theorem: (cosθ + sinθ)n = cos(nθ) + isin(nθ).
Thus, (1 + i)18 =
=
= 512i
问题 4:展开 (-√3 + 3i) 31 .
解决方案:
Here, r = , θ = 2π/3
The polar form of (-√3 + 3i) =
According to De Moivre’s Theorem: (cosθ + sinθ)n = cos(nθ) + isin(nθ).
Thus, (-√3 + 3i)31=
=
问题 5:展开 (1 – i) 10 。
解决方案:
r = , θ = π/4
The polar form of (1 – i) =
According to De Moivre’s Theorem: (cosθ + sinθ)n = cos(nθ) + isin(nθ).
Thus, (1 – i)10 =
=
= 32 [0 + i(-1)]
= 32 (-i)
= -32i
问题 6. 化简 (1 + √3i) 6 .
解决方案:
Modulus of (1 + √3i)6 =
Argument = tan-1(√3/1) = tan-1(√3) = π/3
⇒ Polar form =
Now, (1 + √3i)6 =
As per DeMoivre’s theorem, (cos x + isinx)n = cos(nx) + isin(nx).
⇒
=
= 64(1 + 0)
= 64
问题 7. 简化 i √3 。
解决方案:
Modulus = r = = 1
Argument = tan-1[1/0] = π/2
Polar Form = r[cosθ + isinθ] =
Now, i^{√3} =
As per DeMoivre’s theorem: (cosθ + isinθ)n = cos(nθ) + isin(nθ).
⇒
=