The limits change from a variable in k to a variable in L. Then the summation sign is removed using some identity. I don't know how any of these two steps was carried out.
The limits change from a variable in k to a variable in L. Then the summation sign is removed using some identity. I don't know how any of these two steps was carried out.
Thanks for any explanations.
Substitute . and .
To evaluate the new sum you're expected to recognise that you have a geometric series (the common ratio is r = 0.9) and you're expected to know the formula for the sum of a geometric series.