`a)` $\\$ `(x^2)/4 - xy + y^2 = (x/2)^2 - 2 . x/2 . y + y^2 = ( x/2 - y )^2` $\\$ `b)` $\\$ `2x^2 -sqrt(2) xy + 1/4y^2 = 2x^2 – 2 . sqrt(2)x . 1/2y `$\\$`= ( sqrt(2)x - 1/2y )^2 ` $\\$ `c)` $\\$ `3x^2 - xy + (y^2)/12 = 3x^2 - 2 . sqrt(3)x . y/(2sqrt(3)) + (y^2)/12`$\\$` = ( sqrt(3)x + y/(2sqrt(3)))^2`