Cho hai số phức

Câu hỏi: Cho hai số phức ${\mathrm{z}}_1={\mathrm{r}}_1\left({\mathrm{cos\phi }}_1+{\mathrm{isin\phi }}_1\right),$${\mathrm{z}}_2={\mathrm{r}}_2\left({\mathrm{cos\phi }}_2+{\mathrm{isin\phi }}_2\right)$. Khi đó:

A. ${\mathrm{z}}_1{\mathrm{z}}_2=$${\mathrm{r}}_1{\mathrm{r}}_2\left[\cos \left({\mathrm{\phi }}_1{\mathrm{\phi }}_2\right)+\mathrm{isin}\left({\mathrm{\phi }}_1{\mathrm{\phi }}_2\right)\right]$

B. ${\mathrm{z}}_1{\mathrm{z}}_2=$${\mathrm{r}}_1{\mathrm{r}}_2[\cos \left({\mathrm{\phi }}_1+{\mathrm{\phi }}_2\right)$$+\mathrm{isin}\left({\mathrm{\phi }}_1+{\mathrm{\phi }}_2\right)]$

C. ${\mathrm{z}}_1{\mathrm{z}}_2=$${\mathrm{r}}_1{\mathrm{r}}_2[\sin \left({\mathrm{\phi }}_1{\mathrm{\phi }}_2\right)$$+\mathrm{icos}\left({\mathrm{\phi }}_1{\mathrm{\phi }}_2\right)]$

D. ${\mathrm{z}}_1{\mathrm{z}}_2=$${\mathrm{r}}_1{\mathrm{r}}_2[\sin \left({\mathrm{\phi }}_1+{\mathrm{\phi }}_2\right)$$+\mathrm{icos}\left({\mathrm{\phi }}_1+{\mathrm{\phi }}_2\right)]$