AD=AB/2+BC
AB=λAM+μAN
=λ(AD+AB/4)+μ(AB+BC/2)
=λ(3AB/4+BC)+μ(AB+BC/2)
=(3λ/4+μ)AB+(λ+μ/2)BC
∴3λ/4+μ= 1; λ+μ/2=0
∴λ=-4/5; μ=8/5
∴λ+μ=4/5