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Answer 1 · 2018-04-09T23:29:34

#+-5i#

Remember: #sqrt-1=i# and #sqrt(ab)=sqrt(a*b)#

25 is a perfect square (#5^2=25#, #(-5)^2=25#), separate that from -1

#+-sqrt(25*-1) rarr sqrt-1# is #i#

#+- 5i rarr# The #+-# symbol shows that the answer can be negative or positive

Answer 2 · 2018-04-09T23:31:48

#+-5i#

Remember #i=sqrt(-1#

So:
#+-sqrt(-25)=#

#+-sqrt(-1*25)=#

#+-isqrt(25)=#

#+-5i#