1.y=(4+x)(4-x)2.5000乘以（1+X）的二次方3.D7.设长是x,靠墙,宽是（30-x）/2面积S=x*（30-x）/2=15x-0.5xx=-0.5(x-15)^2+225/2x=15,面积最大112.5平方米8.当S=85时,85=1.8t+0.064t2,0.064t2+1.8t-85=0,解得t1=25,t2=...