y=5000+(90000-5000x)×0.4%
y=5000+(90000-5000(x-2))0.4%
房款:5000(x-1)=5000x-5000;利息:120000-30000-5000(x-2)0.4%=90040-20x;y=房款+利息=4080x+85040.(自己可以求一下取值范围)