Answer:
Nozzle is a device made to control the the flow rate of fluid, as it exits or enters an enclosed chamber.
Nozzle can be convergent, made to accelerate the flow of fluid or divergent, made to reduce the flow of fluid.
A de Laval nozzle has covergent and divergent nozzle, abbrevated as CD Nozzle.
Step-by-step explanation:)
By applying principle of conservation of mass, as follows
m = p * V * A ( where, p = density; V = velocity of fluid surrounding the body of the chamber; A = area at exit of nozzle and m = mass of flow rate
By taking dimension of the converging section as a cone
The curved area of a cone, A = π*r*l ( where r = throat radius; l = length of the CD section of the nozzle)
Hence, m = p * V * π * r * l
∴ lenght of CD, l = m/p * V *π* r
b) The throat radius of nozzle, r = m/p* V *π* l
is a factor of a polynomial, then 
is a root of this polynomial.