#### Answer

The diagonals have an equal length when the figure is a rectangle.

#### Work Step by Step

We place the rectangle at the origin, giving it length a for its short sides and length b for its long sides. Using the Pythagorean theorem, we obtain:
$(d_1)^2 = a^2 + b^2 \\ (d_2) ^2 = a^2 +b^2$
Combining these gives:
$(d_1)^2 = (d_2)^2 \\ d_1 = d_2$
Hence, the diagonals are equal in length.