COOLING \[=\FRAC{d\theta }{dt}=\] slope of the curve. At \[P,\frac{d\theta }{dt}=\TAN {{\phi }_{2}}=k({{\theta }_{2}}-{{\theta }_{0}})\], where k=constant. At \[Q\frac{d\theta }{dt}=\tan {{\phi }_{1}}=k({{\theta }_{1}}-{{\theta }_{0}})\] \[\Rightarrow \frac{\tan {{\phi }_{2}}}{\tan {{\phi }_{1}}}=\frac{{{\theta }_{2}}-{{\theta }_{0}}}{{{\theta }_{1}}-{{\theta }_{0}}}\]