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-z)(1/3+z)-1/9=1/9-z²-1/9=-z²
4) -16/49+(4/7-d)(d+4/7)=-16/49+(4/7-d)(4/7+b)=-16/49+16/49-d²=-d²
5) (0,9-a)(a+0,9)-a(1+a)=(0,9-a)(0,9+a)-a(1+a)=0,81-a²-a-a²=0,81-2a²-a
6) K(5-k)+(1,2+k)(k-1,2)=k(5-k)+(k+1,2)(k-1,2)=5k-k²-1,44=5k-1,44
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Answers & Comments
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-z)(1/3+z)-1/9=1/9-z²-1/9=-z²
4) -16/49+(4/7-d)(d+4/7)=-16/49+(4/7-d)(4/7+b)=-16/49+16/49-d²=-d²
5) (0,9-a)(a+0,9)-a(1+a)=(0,9-a)(0,9+a)-a(1+a)=0,81-a²-a-a²=0,81-2a²-a
6) K(5-k)+(1,2+k)(k-1,2)=k(5-k)+(k+1,2)(k-1,2)=5k-k²-1,44=5k-1,44