Every diagonal in a parallelogram divides it into congruent triangles. 

In parallelogram $ABCD$, its diagonals $AC$ and $BD$ intersect at $O$. If $AO=5 \mathrm{cm}$ then $AC=$ _____ $\mathrm{cm}$. 

In a rhombus $ABCD$, its diagonals intersect at '$O$'. Then $\angle AOB=$ _____ degrees. 

$ABCD$ is a parallelogram, then $\angle A-\angle C=$ _____ degrees. 

In a rectangle $ABCD$, the diagonal $AC=10 \mathrm{cm}$ then the diagonal $BD=$ _____ $\mathrm{cm}$.

In a square $ABCD$, the diagonal $AC$ is drawn. Then $\angle BAC=$ _____ degrees.