# Math Help - Clarification please

I don't understand how the first question (above the line) is correct, while the second one is not and it should be the solution on the right.

2. Hi,

The first one is correct, because when you differentiate (x+1), you got 1, whereas when you got to differentiate (3x^3), you have 9x², which should appear in f '(x).

$(u^n(x))' = u'(x).n.u^{n-1}(x)$

3. Because you have a composite function and you didn't use the chain rule. either use the chain rule for the second question or using the rules of indices.

4. Originally Posted by bobak
Because you have a composite function and you didn't use the chain rule. either use the chain rule for the second question or using the rules of indices.
We haven't learned any of those yet! Only power, product, and quotient rules!!

5. Originally Posted by NAPA55
We haven't learned any of those yet! Only power, product, and quotient rules!!
well, in that case, you should have went for the "And it should be this:" method