1. (5+b)(b-5)-b² = (b+5)(b-5)-b² = b²-25-b²=-25
2. c²+(9-c)(9+c) = c²+81-c² = 81
3. (1/3-x)(1/3+x)-1/9 = 1/9-x²-1/9 = -x²
4. (-16/49+(4/7-D)(D+4/7) = -16/49+(4/7-D)(4/7+D )= -16/49+16/49-D² = -D²
5. (0,9-a)(a+0,9)-a(1+a) = (0,9-a)(0,9+a)-a-a² = 0,81-a²-a-a² = -2a²-a
6. (k(5-k) +(1,2+k)(k-1,2) = 5k-k²+(k-1,2)(k-1,2) = 5k-k²+k²-1,44 = =5k-1,44