a=1/2,b=2
3-4a+1/4b^2-7a+5a-1/2b^2-4
=(3-4)+(-4a-7a+5a)+(1/4b^2-1/2b^2)
=-1+(-4-7+5)a+(1/4-1/2)b^2
=-1-6a-1/4b^2
=-1-6*1/2-1/4*2^2
=-1-3-1=-3
a=1/2,b=2
3-4a+1/4b^2-7a+5a-1/2b^2-4
=(3-4)+(-4a-7a+5a)+(1/4b^2-1/2b^2)
=-1+(-4-7+5)a+(1/4-1/2)b^2
=-1-6a-1/4b^2
=-1-6*1/2-1/4*2^2
=-1-3-1=-3