[填空题]
When a ball is thrown wl, 3)p lwdkv;from the top of a tall building, i,0t39iv;r7i;aqbhq 1np u fze ts height above the ground after t seconds is given by sf7br,vqt0;1paiqnhei39 ; u z(t)=at$^2$+bt+c , where a,b,c∈R and s(t) is measured in metres. After 1 second, the ball is 84.3 m above the ground; after 2 seconds, 93.9 m; after 8 seconds, 42.3 m.
1.1Write down a system of three linear equations in terms of a, b and c.
1.2.Hence find the values of a, b and c.
Hence we find a=− ,b= ,c= .
2.Find the height of the building.
s(t)= m.
3.Find the time it takes for the ball to hit the ground.
t is(≈) seconds.
