Final answer:
The equivalent expression for √-125 is 5i√5,(option b) by using the imaginary unit i and simplifying the square root of 125.
Step-by-step explanation:
To find the equivalent expression for √-125, we need to understand that the square root of a negative number involves the imaginary unit i, where i is defined as √-1. Using this information, we can factor out -125 as (-1)(125) and take the square root of each part separately. The square root of -1 becomes i, and the square root of 125 can be further simplified. Since 125 = 53, its square root is 5√5. Combining these parts gives us 5i√5, which is the square root of -125 expressed with an imaginary unit. Therefore, the correct choice is option b: 5i√5.