11. 4 -a² -2ab-b² =4 -(a² +2ab+b²)= 2² -(a+b)² =(2-a-b)(2+a+b).
---
12. 1 -m² +2mn -n² =1 -(m² -2mn +n²) =1² -(m-n)² =(1-m+n)(1+m-n).
---
13. 1-p² -2pq-q² =1² -(p+q)² =(1-p-q)(1+p+q).
---
14. 4a² -20ab+25b² -36 = ( (2a)² -2*2a*5b+(5b)² ) -6² =
(2a-5b)² -6² =(2a-5b -6)(2a-5b+6).
---
15.16m² -8mn +n² -49 =( (4m)² -2*4m*n+n² ) -7² =
(4m -n)² -7² =(4m -n -7 )(4m -n)+7).