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**Eenhoorn** The cost, £*P *million, of laying one kilometre of pipe for a water main(=supply) is calculated by means of the formula

*P=*4000/9a+4a

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(a)=4000/9a+4a

P^1(a)=4000/9a^-1+4a^-1 next P^1(a)=0

4000/9a^-1+4a^-1=0 then I'm lost