The weather in Yong 'an City, Fujian Province

(1) Let the analytical formula of AB be y=kx+b,

Y = ∫ a (0,3) on kx+b, (1, 1.8),

∴b=3k+b= 1.8，

The solution is k =? 1.2b=3，

∴y=- 1.2x+3.

When x=5, y =- 1.2× 5+3 =-3.

The temperature at 5 o'clock is-3℃;

(2)∵ quadratic function y=-x2+mx+n passes through points B(5, -3) and C(8, 6)

∴? 25+5m+n=？ 3? 64+8m+n=6

The solution is m = 16n =? 58

∴y=-x2+ 16x-58；

(3) When y=0,

From y=- 1.2x+3, x 1=2.5.

By y=-x2+ 16x-58

The solution is x2=8-6(8+6 is truncated);

X2-x 1 = 8-6-2.5 ≈ 3.05 > 3, anti-freezing measures are needed.