Substitute (1, 1 18) and (2, 1 16) to obtain.
k+b = 1 182k+b = 1 16
The solution is k =? 2b= 120
Therefore, the analytic function between the sales volume of P pieces and the sales days of X is p =-2x+120;
(2) When 1 ≤ x < 25
y=(60+x-40)(-2x+ 120)
=-2x2+80x+2400，
When 25≤x≤50,
y =(40+ 1 125 x-40)(-2x+ 120)
= 135000 x-2250；
(3) when 1 ≤ x < 25
y=-2x2+80x+2400，
=-2(x-20)2+3200，
∵-20,
∴ 135000x decreases with the increase of x,
When x=25, 135000x is the largest.
Therefore, when x=25, y= 135000x-2250 has the maximum value y2, and Y2 = 5400-2250 = 3 150.
∫y 1 > y2
On the 20th day of these 50 days, the supermarket made the biggest profit, with the biggest profit of 3,200 yuan.