The second term of the expansion is
.
Solution:
Given expression:

To find the second term of the expansion.

Using Binomial theorem,

Here, a = a and b = –b

Substitute i = 0, we get

Substitute i = 1, we get

Substitute i = 2, we get

Substitute i = 3, we get

Substitute i = 4, we get

Therefore,



Hence the second term of the expansion is
.
On the 21st day, he sprints 205 yards.
Is that what you meant for the problem
or do u want to know how many yards he sprinted total for the 21 days
If it is,
he sprinted a total of 3390 yards