`a)225a^4-1/81b^4`
`=(15a^2)^2-(1/9b^2)^2`
`=(15a^2-1/9b^2)(16a^2+1/9b^2)`
`b)1/64x^6+64y^6`
`=(1/4x^2)^3+(4y^2)^3`
`=(1/4x^2+4y^2)(1/16x^4-x^2y^2+16y^4)`
`c)216x^9-1/216y^9`
`=(6x^3)^3-(1/6y^3)`
`=(6x^3-1/6y^3)(36x^6+x^3y^3+1/36y^6)`
`2)(2x-1)^2+(3x-1)^2+2(2x-1)(3x-1)=0`
`<=>(2x-1+3x-1)^2=0`
`<=>(5x-2)^2=0`
`<=>5x-2=0`
`<=>5x=2`
`<=>x=2/5`
`=>S={2/5}.`