if ( x >= xc )
	y = k0+k1*(x-xc)+k2*(x-xc)^2+k3*(x-xc)^3;
else
	y = k0+k1*(x-xc)+k2*(x-xc)^2+k3*(x-xc)^3+(k4+k5*x+k6*x^2+k7*x^3);

=============================

elseУyֵһҲСk0+k1*(x-xc)+k2*(x-xc)^2+k3*(x-xc)^3	Ϊʹƽٽ㴦ֵһµģԾ

====================

k0         	3.93313994552033
k1         	-0.707166468195573
k2         	-1.96173108968778
k3         	-0.724535375516146
k4         	3.89516558764811
k5         	-3.12074272869209
k6         	0.630361482478928
xc         	2.50176464270392
====================