9514 1404 393Answer:
21·a^5·b^2
Step-by-step explanation:
The k-th term is ...
C(7, k)·a^(7-k)·b^k . . . . . where k counts from 0 to n
Then k=2 for the third term. The coefficient of it is ...
C(7, 2) = 7!/(2!(7-2)!) = 7·6/(2·1) = 21
The third term is ...
21·a^5·b^2
