(1)(5-2x)(2x-5);
=-(2x-5)(2x-5)
=-(4x^2-20x+25)
=-4x^2+20x-25
(2)(a-2b+3c)²
=[(a-2b)+3c]^2
=(a-2b)^2+6c(a-2b)+9c^2
=a^2-4ab+4b^2+6ac-12bc+9c^2