Hi, can someone explain to me the fundamentals of the reduction in step 2 of this symbolic convolution example?

 

Which line are you calling "step 2"?

 

Which line are you calling "step 2"?

5th

 

5th

The fifth line is merely applying the distributive law of multiplication over addition.

(a+b)c = ac + bc

followed by breaking the summation into to parts

sum_over_limits(a - b) = sum_over_limts(a) - sum_over_limits(b)