Find the diagonal of a rectangular parallelepiped if its dimensions are 8; 5; 6.

We know well from the school geometry course that the length of a diagonal in such a geometric figure can be calculated using the following formula:
d ^ 2 = a ^ 2 + b ^ 2 + c ^ 2, where a, b and c are the same dimensions of a rectangular parallelepiped.
Let us find out what value will correspond to the length of the diagonal in our rectangular parallelepiped, when it is precisely known from the condition of our problem that its dimensions are 8, 5 and 6, respectively:
d ^ 2 = 8 ^ 2 + 5 ^ 2 + 6 ^ 2;
d ^ 2 = 125;
d1 = 5√5;
d2 = -5√5 (does not fit).
Answer: 5√5 cm.