Hello!
To solve this we can use the Pythagorean Theorem, a²+b²=c², where a and b represent the legs and c represents the hypotenuse. We will solve below.
1²+3²=c²
1+9=10
We have 10=c². We can isolate c by finding the square root of 10.
√10≈3.16
Therefore, the other side could be 3.16.
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Note that 3 could also be the hypotenuse. We could solve the equation the other way around to see what the other side could be (it could be the hypotenuse or a leg).
9-1=8
√8≈2.83
Therefore, the length of the third side could be either 3.16 or 2.83.
I hope this helps!