We need to find θ in radians (see Trigonometric Functions of Any Angle if you need a reminder about reference angles) and r.
`alpha=tan^(-1)(y/x)` `=tan^(-1)(5/1)` `~~1.37text( radians)`
[This is `78.7^@` if we were working in degrees.]
Because our angle is in the second quadrant, we need to apply:
`theta = pi - 1.37 ~~1.77`
`=sqrt( (-1)^2 + (5)^2 )`
` ~~ 5.10`
So `-1 + 5j` in exponential form is `5.10e^(1.77j)`
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