y=4^x/4^(1/2)-3*2^x+5=(1/2)*4^x-3*2^x+5令a=2^x,则4^x=a^20<=x<=22^0<=2^x<=2^2所以1<=a<=4y=(1/2)a^2-3a+5=(1/2)(a-3)^2+1/21<=a<=4所以a=3,y最小=1/2a=1,y最大=5/2