`P = ( x/4 )^3 + ( y/2 )^3`
`<=> P = ( x/4 + y/2 )[ ( x/4 )^2 - (xy)/(4.2) + ( y/2 )^2 ]`
`<=> P = (x+2y)/4 . ( x^2/16 - (xy)/8 + y^2/4 )`
`<=> P = 0/4 . ( x^2/16 - (-6)/8 + y^2/4 )`
`<=> P = 0 . ( x^2/16 + 3/4 + y^2/4 )`
`<=> P = 0`
Áp dụng hằng đẳng thức : `a^3 + b^3 = ( a + b )( a^2 - ab + b^2 )`