If α,β,γ,δ are in A.P. and ∫20f(x)dx=−4 wheref(x)=∣∣ ∣ ∣∣x+αx+βx+α−γx+βx+γx−1x+γx+δx−β+δ∣∣ ∣ ∣∣then the common difference d is: