mathematicians

(Original post by sapah)
A company expects to sell 20000 computers in the first year if the price of each computer is £650.
Let x represent the number of £’s by which the price has decreased.
a Write an expression for the price, p, of one computer, in the form p = a + bx.
(1)
The company expects to sell an additional 50 computers every time the price decreases by £1.
b Write an expression for the number of computers sold, C, in the form C = d + ex.
(1)
Revenue is defined by the formula,
revenue = (number of computers sold) × (cost of one computer)
c Write an equation for revenue, r, in the form A – B(x – C)
2
, where A, B and C are constants to be
found.
(4)
The company wishes to maximise the revenue.
d Using your answer to part c, or othwerwise, state the price the company should charge for each
computer and the revenue they will attain.

Alsoooooooooooooooooooo
Martin tried to find all the solutions of 4sin^2xcos^2x -cos^2x=0
His working:
4sin^2xcos^2x -cos^2x=0
4sin^2xcos^2x= cos^2x
4sin^2x=1
Sin^2x=1/4
Sinx=1/2
X=30°,150°

Martin did not find all the correct solutions because he made two errors

Identify the two errors and explain the consequences of each error