`#Sad`
`\text{→Chứng minh}`
`a)`
`x^2+y^2 = (x+y)^2-2xy`
`\text{Ta có:}`
`\text{VT}` `= x^2+y^2`
`= (x^2+2xy+y^2)-2xy =` `\text{VP}` `\text{(→đpcm)}`
`b)`
`(a+b)^2-(a-b)(a+b) = 2b(a+b)`
`\text{Ta có:}`
`\text{VT}` `= (a+b)^2-(a-b)(a+b)`
`= (a+b)^2-(a^2-b^2)`
`= a^2+2ab+b^2-a^2+b^2`
`= 2ab+2b^2`
`= 2b(a+b) =` `\text{VP}` `\text{(→đpcm)}`