# Homework Help: Einstein Notation - Not sure if my answers are right

1. Feb 15, 2013

### unscientific

1. The problem statement, all variables and given/known data

Questions 11 and 12 specifically..

2. Relevant equations

3. The attempt at a solution
11(a)

11(b)

12(a)

12(b)

12(c)

12(d)

(I did the last part of 12(d) by normal vector methods and got 2a instead...which is the correct answer)

Last edited: Feb 15, 2013
2. Feb 16, 2013

### vela

Staff Emeritus
An index shouldn't appear more than twice. For the first one, for example, you should have
$$\frac{\partial}{\partial r_i} (r_j r_j)^{n/2}$$

3. Feb 16, 2013

### unscientific

Thanks for your reply!

Also, I can't see what's wrong with my final answer in part 12(d)...i think the last line is the cross product of ∂j and r which equates to zero??

I think I have figured out every question other than part 12(d)

Last edited: Feb 16, 2013
4. Feb 16, 2013

### vela

Staff Emeritus
Hint: $\partial_i r_j = \delta_{ij}$. You can use that in several of the problems. In 12b, you need to use the product rule.

5. Feb 16, 2013

### unscientific

thanks i think i've sorted it out!

6. Feb 16, 2013

### WannabeNewton

It is not a summation. It is just the statement that the partial of the jth coordinate function with respect to the ith coordinate function is just the kronecker delta.

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