The cost, £P million, of laying one kilometre of pipe for a water main(=supply) is calculated by means of the formula
where a is the cross-sectional area of the pipe in square metres.
a: What is the cross-sectional area of the most economical pipe to use? (Answer to the nearest tenth of a square metre)
b:Calculate the minimum cost of laying one kilometre of pipe (Answer to the nearest £ million)
I do not want to discuss million or billion.....
English homework is somewhere else!
to a: I know I have to work with the derivative of the function
P^1(a)=4000/9a^-1+4a^-1 next P^1(a)=0
4000/9a^-1+4a^-1=0 then I'm lost