viết các biểu thức sau dưới tổng hai bình phương 1,$a^{2}$ -4ab+5$b^{2}$ -4bc+4$c^{2}$ 2,9$x^{2}$ +25-12xy+2y^2-10 3,$x^{2}$ +4$y^{2}$ +4x-4y+5 4,$x^

`1)a^2-4ab+5b^2-4bc +4c^2`

`= a^2-4ab+4b^2+b^2-4bc +4c^2`

`=( a^2-4ab+4b^2)+(b^2-4bc +4c^2)`

`=(a-2b)^2+(b-2c)^2.`

`2) 9x^2 +25-12xy+2y^2-10`

`= (9x^2 -12xy+4y^2)-2x^2+15.`

`= (3x-2y)^2-2x^2+15.`

`3)x^2 +4y^2 +4x-4y+5`

`=(x^2+4x+4)+(4y^2-4y+1)`

`=(x+2)^2+(2y-1)^2.`

`4) x^2+y^2+2y-6x+10`

`=(x^2+2y+1)+(y^2-6y+9)`

`=(x+1)^2+(y-3)^2.`

`5)y^2+2y+5-12x+9x^2`

`=(y^2+2y+1)+(9x^2-12x+4)`

`=(y+1)^2+(3x-2)^2.`

`6)10-6x+12y+9x^2+4y^2`

`=(9x^2-6x+1)+(4y^2+12y+9)`

`=(3x-1)^2+(2y+3)^2.`