factoring issue (related to calc)

[Good_Birdy]

Originaly I was finding when a ball is at 117ft when modeled by the eq:
s(t)=-16^2+128t+5

the book tells me to factor and set the equation equal to 117 so:
-16^2+128t+5=117
-16(t^2-8t+7)=0 <--- I don't understand how they got here.
-16(t-1)(t-7=0

Can anyone help me?

 

[galactus]

the book tells me to factor and set the equation equal to 117 so:
-16^2+128t+5=117
-16(t^2-8t+7)=0 <--- I don't understand how they got here.
-16(t-1)(t-7=0

Can anyone help me?

They just subtracted 117 from both sides and factored out -16

 

[Good_Birdy]

I don't see where/how they subtracted it from both sides if you don't mind elaborating please.

...and i copied that wrong it is:
-16t^2+128t+5

 

[skeeter]

-16t^2 + 128t + 5 = 117

subtract 117 from both sides ...

-16t^2 + 128t + 5 - 117 = 117 - 117

-16t^2 + 128t - 112 = 0

factor out -16 from every term ...

-16(t^2 - 8t + 7) = 0

factor the quadratic factor ...

-16(t - 1)(t - 7) = 0

set each variable factor equal to 0 ...

t - 1 = 0 ... t = 1
t - 7 = 0 ... t = 7

the ball is at 117 ft above the ground at t = 1 second (on the way up), and at t = 7 (on the way back down).

 

[Good_Birdy]

yay fabulousa!

Wonderful explenation, thank you *very* much!