Volume of a hemisphere is half of the volume of sphere?

The mid-value of a class interval is $42$ and the class size is $10$. The lower and upper limits.

Factorise: $9x^2+16y^2+25z^2+24xy-40yz-30xz$

Prove that the bisectors of the angles of a linear pair are at right angle.

In the given figure, $PS$ and $RT$ are medians of $∆PQR$ and $SM\Vert RT$.

Prove that $QM=\frac{1}{4}PQ$.