1. (а-4)(а+4)+16 = а^2-16+16 = a^2
2. (х-5)^2+(х-5)(х+5) = х^2-10х+25+х^2-25 = 2х^2-10х = х^2-5х
3. 2а(а+4)+(а-3)(а-4) = 2а^2+8а+а^2-4а-3а+12 = 3а^2+а+12
4. 2(х+6)^2-(20х+70) = 2х^2+24х+72-20х-70 = 2х^2+4х+2 = х^2+2х+1
5. (1-5х)(1+5х)-(3х-1)^2 = 1-25х^2-9х^2+6х-1 = -36х^2+6х