total cost for a bus to run for one hour is 90+(v²/10) .v is speed of the bus..

take "C" as the cost for bus to run 1Km.What is the speed(v ) need for bus to make the cost minimum

Here what i did:

C=[ 90+(v²/10) ] v -this is the cost for bus to run one hour.

Dc/Dv= 90 + 3v³/10

set to zero and solve

v=√-300 O.o

answer has to be 30 as books says

help

thanks