$7 \text { cm}$ ?

What is the length of the diagonal of a cube of side**
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Using Pythagorean Theorem

let y be the side of the cube, x be the diagonal of a face of the cube and a the diagonal of the cube.

y^2+y^2=x^2

2(y^2)=x^2

x^2+y^2=a^2

(2(y^2))+(y^2)=a^2

3(y^2)=a^2

a=y(sqrt(3))

a=7(sqrt(3))

Looking through the choices, only (sqrt((1078/3)x3)) is equal to (7(sqrt(3)))

so it's the answer

(sqrt((1078/3)x3))