need to multiply
(x^2-1)^4
I get
-20x^9+32x^7-12x^5
Then i need to differentiate this 4 times
d^4/dx^4 (-20x^9+32x^7-12x^5)
I get
-10080x^6+6720x^4-720x^2
Is this right?
Think about this. Your highest power in the expansion will be . So you can't be right.
By the binomial theorem:
You have and :
Thus
(I'd recommend making sure you know how to do it the long way as well. It's good practice.)
And make sure you take 4 derivatives. You only took 3.
-Dan