Mod-01 Lec-07 Introduction to Finite Element Method 

r subscript j , l implies partial differentiation of r subscript j with l.So according to rule of partial differentiation when partially differentiating a function with respect to a variable all remaining variables are constant so when j not equal to l differentiation yields zero and when j = l differentiation yields coefficient of component of r in j direction.Here coeffcient is taken as 1.So answer is 1.

r subscript j , l implies partial differentiation of r subscript j with l.So according to rule of partial differentiation when partially differentiating a function with respect to a variable all remaining variables are constant so when j not equal to l differentiation yields zero and when j = l differentiation yields coefficient of component of r in j direction.Here coeffcient is taken as 1.So answer is 1.

Thank you!

***** The only possibilily for that term not to be equal to zero is for l to be equal to j

Abhijit has answered below. Just go through

r1 s unit vector is x1

But it simplifies equations a lot. In graduate level courses like "Micromechanics" indicial notation serves as a very important tool.