A lens of focal length 15.0 cm forms an image 77.5 cm from the
lens. What was the object
distance?
(Unit = cm)

i think it because equal matrices have equal dimensions, only square matrices can be symmetric. for all indices and . every square diagonal matrix is symmetric, since all off-diagonal elements are zero. in linear algebra, a real symmetric matrix represents a self-adjoint operator over a real inner product space.

3. dark matter

empty space could also be a simple vacuum