The  V-I graph for a conductor temperatures T1  and T2  are as shown in the fig.T2− T1 is proportional to

The slope of V – I graph gives resistance R1= tan θ , R2= tan 90-θ = cot θ Let say α is temperature coefficient R1=R0 1+αT1 , R2=R0 1+αT2 ⇒R2- R1 = R0 α T2−T1 ⇒cot θ - tan θ =  α  T2-T1 ⇒ T2-T1  ∝ 2cos2 θ-sin2 θ2cos θsinθ ⇒ T2-T1  ∝  2cos2θsin2θ ⇒ T2-T1  ∝ cot2θ