# Final in 2 hours, Zeros, amplitude, period etc

• Dec 9th 2008, 10:06 AM
Garface
Final in 2 hours, Zeros, amplitude, period etc
1. find the zeros of $(x^2 + 2x - 7) (x^3 + 4x^2-21x)$

I got 0,-7,3 was hoping someone could do it and see if they get the same answer.

2. $x^4 + 6x^2 + 9$
I got no real zeros

3. Find the amplitude, period and phase shift. $y = cos(2t + pi/2)$
I got: amplitude = 1, period = pi, phase shift = pi/2

Any help on any of them is EXTREMELY welcome. Thanks a million
*edit* just put this in the pre calc help area seeing as I think I will get replys quicker and then noticed in the rules not to double post and I dont want to be a pest there a delete button for the entire thread?
• Dec 9th 2008, 11:46 AM
running-gag
Quote:

Originally Posted by Garface
1. find the zeros of $(x^2 + 2x - 7) (x^3 + 4x^2-21x)$

I got 0,-7,3 was hoping someone could do it and see if they get the same answer.

2. $x^4 + 6x^2 + 9$
I got no real zeros

3. Find the amplitude, period and phase shift. $y = cos(2t + pi/2)$
I got: amplitude = 1, period = pi, phase shift = pi/2

Any help on any of them is EXTREMELY welcome. Thanks a million
*edit* just put this in the pre calc help area seeing as I think I will get replys quicker and then noticed in the rules not to double post and I dont want to be a pest there a delete button for the entire thread?

Hi

1. 0,-7,3 are the zeros of $x^3 + 4x^2-21x$ but you have also to find the zeros of $x^2 + 2x - 7$

2. OK since $x^4 + 6x^2 + 9 = (x^2+3)^2$

3. OK
• Dec 9th 2008, 11:59 AM
Garface
For number one I didnt think there were any real zeros of the left hand side of the equation or at least not any that i could find. Graphing it I noticed that the parabola did cross the x axis but i tried using those numbers and it did not come out to exact zeros.
• Dec 9th 2008, 12:11 PM
Greengoblin
Have you tried the quadratic equation?

$x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}$

For a quadratic of the form:

$ax^2+bx+c$