## Precalculus (6th Edition) Blitzer

The joke tells us to multiply the imaginary number with $i$. The imaginary number is the product of a real number with $i$. For example, $5i$ The imaginary unit $i$ is the solution of the equation${{x}^{2}}=-1$ and its value is equal to$\sqrt{-1}$. The product of imaginary number with $i$ gives a real number. \begin{align} & \left( \text{Imaginary number} \right)\cdot i=\left( 5i \right)\cdot i \\ & =5{{i}^{2}} \\ & =5\left( -1 \right) \\ & =-5 \end{align} If the imaginary number is multiplied with $i$ then the number obtained will be a real number and hence, it is possible to dial the number. Therefore, the multiplication of the imaginary number with $i$ gives a real number.