An electric charge q₁ moving with velocity v₁ produces a magnetic induction B given by

B = μ₀/4π q₁ (v₁ x r)/r² (mks units),

where r is a unit vector that points from q₁ to the point at which B is measured.

a) Show that the magnetic force exerted by q₁ on a second charge q₂, velocity v₂, is given by the vector triple product
F₂ = μ₀/4π q₁q₂/r² v₂ x (v₁ x r).