How to Correctly Find the Derivative of a Trinomial

[Ry122]

Im don't know how to find the derivative of this trinomial:
y=.2x(x-5.1)(x-9.1)
I tried multiplying the 2nd and 3rd terms by the first one
which results in (.2x^2-1.02x)(.2x^2-9.1x), then with these two terms i used the product rule:
y'=u'v+v'u
The answer i got was y=(.4x-1.02*.2x^2-9.1x)+(.4x-9.1*.2x^2-1.02x)
i checked if this was correct by graphing the derivative against my derivative on my calculator and they didnt match up.

 

[Doc Al]

Im don't know how to find the derivative of this trinomial:
y=.2x(x-5.1)(x-9.1)
I tried multiplying the 2nd and 3rd terms by the first one
which results in (.2x^2-1.02x)(.2x^2-9.1x),

This is incorrect. You can multiply the .2x into the 2nd or the 3rd factor, but not both! (a*b*c does not equal [a*b]*[a*c] = a^2*b*c)

You can also just multiply the entire polynomial out to make taking the derivative even easier.

 

[EugP]

Im don't know how to find the derivative of this trinomial:
y=.2x(x-5.1)(x-9.1)
I tried multiplying the 2nd and 3rd terms by the first one
which results in (.2x^2-1.02x)(.2x^2-9.1x), then with these two terms i used the product rule:
y'=u'v+v'u
The answer i got was y=(.4x-1.02*.2x^2-9.1x)+(.4x-9.1*.2x^2-1.02x)
i checked if this was correct by graphing the derivative against my derivative on my calculator and they didnt match up.

What you did is wrong, you can't multiply both by .2x, only one of them.

--EDIT--

Sorry, I didn't know Doc Al already posted.