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Q7. (a) The cost, C million pounds, of pumping water at a particular hydro-electric station is related to the diameter, d, of the pipe carrying the water by the formula:

C = d/2 + 18/d + 6

Calculate EACH of the following:

(i) the diameter that minimises the cost; (8)

(ii) the minimum cost. (2)

(b) Given: H = 3xy - 2y dy/dx and y = x²- √x

Calculate the value of H when x = 4. (6)