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

`\\ \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

Answer 2

x = -1 and 5

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`