[填空题]
When a ball is thrown from t 6d eb:(tq3 oszbn*ustl u,86dhe top of a tall bu.iu 1x-in zs0iilding, its height above the ground afts-1ii.iuxz 0 ner t seconds is given by s(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.
