A= x(x+5)+3(x+5)+4 =x2+5x+3x+15+4 =x2+8x+19 =x2+2.4.x+16+3=(x+4)2+3
ta thay : (x+4)2>hoac = 0 suy ra Amin khi va chi khi x+4=0 suy ra x=-4
Vay Amin = 3 khi x=-4
B=x2-4x+4+y2-8y+16-14 =(x-2)2+(y-4)2-14
vi (x-2)2 va (y-4)2 lon hon hoac bang 0 suy ra Bmin khi va chi khi (x-2)2=0 va (y-4)2=0
tinh ra nhu cau a (ban tu lam nhe)
vay Bmin=-14 va x=2 va y=4