Question

Help me please please

Answer 1

A)

`h(t)=-16t^2+64t+80\implies 0=-16t^2+64t+80\implies 0=-16(t^2-4t-5) \\\\\\ 0=t^2-4t-5\implies 0=(t + 1)(t - 5)\implies t= \begin{cases} -1~\bigotimes\\ ~~ 5~\textit{\LARGE \checkmark} \end{cases}`

why can't we use -1? well, "t" is seconds above the ground, hmm -1 will be under the ground? how would it do that? well, it can't, so "t" can't be negative, so it can't hit the ground at -1 seconds.

B)

`\stackrel{h(t)}{128}=-16t^2+64t+80\implies 128=-16(t^2-4t-5)\implies \cfrac{128}{-16}=t^2-4t-5 \\\\\\ -8=t^2-4t-5\implies 0=t^2-4t+3\implies 0=(t - 1)(t - 3)\implies t= \begin{cases} 1\\ 3 \end{cases}`

C)

`h(t)=-16t^2+64t+80\implies h(4)=-16(4)^2+64(4)+80\implies h(4)=80`