Chapter 11 - 11.5 - The Binomial Theorem - 11.5 Exercises - Page 813: 49

$a=720$

According to the Binomial Theorem, the 8th, that is, the ($8+1$)th term of $(4x-y)^{10}$ is: $_{10}C_8(4x)^{10-8}(-y)^8=\frac{10!}{(10-8)!\times8!}(4x)^2y^8=(45)(16)x^2y^8=720x^2y^8$ $ax^2y^8=720x^2y^8$ $a=720$