(a) Use Eq. (14), Section 8.3, to show that the force of attraction per unit length between two filamentary conductors in free space with currents I1a_z at x = 0, y = d/2, and I²a_z at x = 0, y = −d/2, is μ₀ I1 I²/(2πd).

(b) Show how a simpler method can be used to check your result.