I believe it is an imaginary number because imaginary numbers are square roots of negative numbers.
The space diagonal will have length ...
... d = √(1² +4² +2²) = √(1 +16 +4) = √21
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This can be found using the Pythagorean theorem. A drawing can help. Find the length of any face diagonal, then use that length as the leg of a right triangle whose hypotenuse is the space diagonal and whose other leg is the edge length not used in the first calculation.
Answer:
Part 1) 
Part 2) 
Part 3) 
The answer is the option A
Step-by-step explanation:
Part 1) Find the measure of side HI
Applying the Pythagoras Theorem

substitute the values and solve for HI




Part 2) Find the measure of angle G
In the right triangle GHI

substitute the values


Part 3) Find the measure of angle I
Remember that
The sum of the interior angles of a triangle must be equal to 180 degrees
so

substitute the values


Answer: can't add an answer, but for the first all 3 angles add up to 180 in a triangle for the second, plug 5 in and also check if all three add up to 180
Step-by-step explanation: