當分母為 虛數數值不等於零的複數, 以共軛作擴分母
例子:
1/(2+3i) = 1/(2 + 3i) x (2 – 3i)/(2 – 3i)
= [1 x (2 + 3i)] / [ (2 + 3i) x (2 – 3i)]
= [2 + 3i] / [ 2×2 -2x3i + 2x3i – 3ix3i]
= [2 + 3i] / [ 4 + 0 – 9(i x i)]
= [2 + 3i] / [ 4 – 9x (-1)]
= [2 + 3i] / [ 4 + 9]
= 2/13 + (3/13) i