B=2(x^2+3/2x+9/16)+7/8
2(x^2+3/4)^2+7/8
vi 2(x+3/4)^2>=
suy ra B>=7/8
dau bang say ra khu va chi khi x+3/4=0 suy ra x=-3/4
vay gia tri nho nhat cua bieu thuc B =7/8 khi x=-3/4
d cau d tung tu tao khong doi hoi vi tao phai lam bai tap ve nha ngay mai roi nhe