1/9x^2-1/25y^2=(1/3x+1/5y)(1/3x-1/5y)x^4-16=(x^2+4)(x^2-4)=(x^2+4)(x+2)(x-2)-9x^2+24xy-16y^2=-(9x^2-24xy+16y^2)=-(3x-4y)^2x^4-18x^2+81=(x^2-9)^2=(x+3)^2(x-3)^24a^4+12a^2b^2+9b^4=(2a^2+3b^2)^2