U wont have to solve these types of equations algebraically :)
Usually the only way to check these types of questions is to do them again (by accurately sketching the 2 graphs again - not too accurate)
$sinx = $\frac{x}{2} \\ \\ \therefore x = 2\sin{x}
Draw out y = sinx from x = -pi to x = 2pi
Draw out y = x from x=-pi to x = 2pi
(a)u just look at the graphs from x = 0 to x = 2pi and count how many intersections
(b)u just look at the graphs from x = -pi to x = pi and count how many...
Who is 'they'?
and yeh i think u just find the volume of the entire sphere... using...
V = \pi \int_a^b y^2 dx
Unless the ________ is rotated around the y-axis, then u find the area of a hemisphere