A ball is thrown from an initial height of 7 feet with an initial upward velocity of 21 ft/s . The ball's height h (in feet) after t seconds is given by the following.

Question

anonymous · Answer

$$h=7+21t-16t ^{2}$$

anonymous · Answer

Find all values of t for which the ball's height is 13 ft.

anonymous · Answer

Hint. All of the information and numbers given to you in the first sentence are irrelevant to answering the question. Only the second sentence and the equation are relevant.

anonymous · Answer

am I substituting t for 13?

anonymous · Answer

13ft is a height. t represents time. no. try again.

anonymous · Answer

do I replace h for 13 so it'd be $$13=7+21t-16t ^{2}$$

anonymous · Answer

?

anonymous · Answer

yes

anonymous · Answer

ok. so then do I subtract 13 from both sides?

anonymous · Answer

that's right.

anonymous · Answer

ok. thennnnn do I factor? how do i solve for t?

anonymous · Answer

oh i see. alright i'll do that

anonymous · Answer

actualy, i would not subtract 13. I'd move everything over to the left side to get
16t^2 - 21t+ 6 = 0

anonymous · Answer

ok. what's next?

anonymous · Answer

And that, I'm afraid, is one ugly quadratic equation! It doesn't even factor according to my calculator..(my calculator can factor quadratics)..so you'll have to use the quadratic formula

anonymous · Answer

ok ok. i can do that, just give me a sec

anonymous · Answer

you should get a ti-89 calculator
i can just type "solve16t^2-21t+6=0,t)" and it spits out the answers. lol

anonymous · Answer

I got 0.8921823261 or 0.4203176739

anonymous · Answer

that's what i got, too. good luck!

anonymous · Answer

oh that's it? that's t?

anonymous · Answer

t = 0.892 seconds and t = 0.420 seconds

anonymous · Answer

yup, that's what I have. Thank you so much!