Chapter 5 - Section 5.7 - Factoring by Special Products - Exercise Set - Page 309: 26

-(x^2-x-10)(x^2+x+10)

Using factoring techniques, then, \begin{array}{l} x^2+20x+100-x^4 \\= (x^2+20x+100)-x^4 \\= (x+10)^2-x^4 \\= [(x+10)-x^2][(x+10)+x^2] \\= (x+10-x^2)(x+10+x^2) \\= (-x^2+x+10)(x^2+x+10) \\= -(x^2-x-10)(x^2+x+10) .\end{array}