Question

what is the vertex of the graph of y=-4(x-1)(x+5)

Answer 1

expand it first

Answer 2

uuuuuuuum (-4x_4)(x+5)

Answer 3

?

Answer 4

I assume _ is +
and then expand (-4x+4)(x+5)

Answer 5

no its - sorry

Answer 6

Oh.. then you were wrong..

Answer 7

y=-4(x-1)(x+5)
= [(-4)x -(-4)(1)] (x+5)
= (-4x + 4)(x+5)

Answer 8

Got it?

Answer 9

ok

Answer 10

but what do i do after that

Answer 11

Now, expand (-4x+4)(x+5)

Answer 12

so combine? them

Answer 13

Expand them...

Answer 14

(-4x+4)(x+5)
= -4x(x+5) + 4(x+5)
= ?

Answer 15

-4x^2-20x+4x+5
-4x^2-16x+5

Answer 16

Nope, the constant term is incorrect.

Answer 17

ummm

Answer 18

(-4x+4)(x+5)
= -4x(x+5) + 4(x+5)
= -4x(x) -4x(5) + 4x + 4(5)
=?

Answer 19

4

Answer 20

and 5

Answer 21

._. Nope....
Show your workings please

Answer 22

._. Perhaps I show it to you...
-4x(x+5) + 4(x+5)
= -4x^2 -20x + 4x +20
= -4x^2 -16x + 20
You need to know what it is 4 times 5, which is equal to 20, for the constant term..
Got it?

Answer 23

ok so the constant term is 20?

Answer 24

Yes... Now, what we need to do is to completing square..
y = -4x^2 -16x + 20
= -4(x^2 +4x) + 20
= -4(x^2 + 4x +4 -4) +20
= -4 (x+2)^2 + (-4)(-4) + 20
= ??

Answer 25

YOu don't need to expand it to find the vertex!! since this quadratic is in "intercept form", identify the 2 x-intercepts! They are 1 and -5... the axis of symmetry is the midpoint of these 2 numbers: -2..... Now, plug -2 into the original equation to determine the vertex!! (-2, 36)

Answer 26

wooooooooooah

Answer 27

There are other ways to find the vertex, but this is the easiest way (for this instance)

Answer 28

:| Haven't thought of that... I'm stupid :(

Answer 29

http://www.mathplane.com/gate_5algebra_ii/quadratics_overview__notes