SOLUTION: Could someone help me with this one; sqrt(-25)sqrt(-9) My answer was 15, but it got marked wrong on my test, but sqrt(-25)=-5 and sqrt(-9)=-3 and -5*-3=15 so I dont really unde

Algebra ->  Algebra  -> Square-cubic-other-roots -> SOLUTION: Could someone help me with this one; sqrt(-25)sqrt(-9) My answer was 15, but it got marked wrong on my test, but sqrt(-25)=-5 and sqrt(-9)=-3 and -5*-3=15 so I dont really unde      Log On

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Question 186786: Could someone help me with this one;
sqrt(-25)sqrt(-9)
My answer was 15, but it got marked wrong on my test, but sqrt(-25)=-5 and sqrt(-9)=-3 and -5*-3=15 so I dont really understand why its wrong.
Found 2 solutions by jim_thompson5910, D13E12:
Answer by jim_thompson5910(21685) About Me  (Show Source):
You can put this solution on YOUR website!
Note: sqrt%28-25%29%3C%3E-5 and sqrt%28-9%29%3C%3E-3 since you CANNOT take the square root of a negative number. Instead, we denote the value i=sqrt%28-1%29 where 'i' is an imaginary number.


So...

sqrt%28-25%29%2Asqrt%28-9%29=sqrt%28-1%2A25%29%2Asqrt%28-1%2A9%29=sqrt%28-1%29%2Asqrt%2825%29%2Asqrt%28-1%29%2Asqrt%289%29=i%2A5%2Ai%2A3=%285%2A3%29%2A%28i%2Ai%29=15i%5E2=15%28-1%29=-15


This means that sqrt%28-25%29%2Asqrt%28-9%29=-15


So you were off by a sign.

Answer by D13E12(2) About Me  (Show Source):
You can put this solution on YOUR website!
Sqrt(-25)= 5i, where i is the sqrt(-1)
sqrt(-9) = 3i , i= sqrt(-1)
now sqrt(-25)sqrt(-9) = 5i*3i=15 (i)^2=15*-1=-15
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