Question

A bird drops a stick to the ground from a height of 80 feet. The equation 0=−16x^2+64x+80 gives the number of seconds that have passed when it hits the ground. After about how many seconds did the stick hit the ground?

I need an answer pretty quickly, and I can't figure out the answer. The answer should look like this, x= so and so

Thanks!!!!!!!

Answer 1

Answer:

x = -1 and 5

Step-by-step explanation:

Solve:  $-16x^2+64x+80=0$

Factor out common value of 16:

$\implies 16(-x^2+4x+5)=0$

Divide both sides by 16:  

$\implies -x^2+4x+5=0$

Change signs:

$\implies x^2-4x-5=0$

Break expression into groups:

$\implies x^2+x-5x-5=0$

$\implies( x^2+x)-(5x+5)=0$

Factor parentheses:

$\implies x(x+1)-5(x+1)=0$

Factor out common term $x+1$:

$\implies (x+1)(x-5)=0$

Therefore,

$x+1=0$  and  $x-5=0$

So $x=-1$ and $x=5$

Answer 2

$\\ \tt\hookrightarrow -16x^2+64x+80=0$

$\\ \tt\hookrightarrow 16x^2-64x-80=$

$\\ \tt\hookrightarrow 16(x^2-4x-5)=0$

• Cancel 16

$\\ \tt\hookrightarrow x^2-4x-5=0$

$\\ \tt\hookrightarrow x^2-5x+x-5=0$

$\\ \tt\hookrightarrow x(x-5)+1(x-5)=0$

$\\ \tt\hookrightarrow (x+1)(x-5)=0$

• x=-1,5