Question

a stone is thrown in the air. after t seconds, it's height , h, above sea level is given by formula h=80+3t-5t^2. Find the valueof t when the stone falls into the sea

Answer 1

hint: when it falls into the sea h = 0

Answer 2

so, 80+3t+5t^2=0

Answer 3

?

Answer 4

yes

Answer 5

yes @n9767 you can solve now

Answer 6

k thanks!!!

Answer 7

but ut doesnt work

Answer 8

which numbers add up to 3 and multiply to 80?

Answer 9

you can't factor this expression but you can use the quadratic formula

Answer 10

factors of 80:
16x5
20x4
40x2
8x10
80x1

Answer 11

how?

Answer 12

ax^2+bx+c
you have to find factors of ac so it equals b

Answer 13

ik but i can't find any

Answer 14

ac = 400

Answer 15

use t = {-b +/- sqrt(b^2-4ac) ]/ 2a

Answer 16

:( yeah i don't think i've learnt that

Answer 17

ac = 400 but i dont think you can get +3 from the factors can u?

Answer 18

nope i just tried

Answer 19

still trying

Answer 20

No I think you have to use the formula

Answer 21

ok - then you can use completing the square - have you learnt that?

Answer 22

no :(

Answer 23

how do you do it?

Answer 24

it sounds interesting

Answer 25

K I will leave now!

Answer 26

correction ac = -400

Answer 27

i havent time to explain now im afraid
try google ' completing the square;

Answer 28

it's fine! i'll do that