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$17-135$ Helium expands in a nozzle from $1$MPa,$500K,$

and negligible velocity to $0\mathrm{.}1$ MPa $.$ Calculate the throat and

exit areas for a mass flow rate of $0\mathrm{.}25k\frac{g}{s},$ assuming the

nozzle is isentropic$.$ Why must this nozzle be converging$-$

diverging? Answers: $3\mathrm{.}51c{m}^2,5\mathrm{.}84c{m}^2$