1) (3x+4)(3x-4)=(3x)^2-4^2=9x^2-16
2) (0,5b-6c)(0,5b+6c)=0,5^2×b^2-36c^2=0,25b^2-36c^2
3) (7x^2-y)(y+7x^2)=(7x^2-y)(7x^2+y)=49x^4-y^2
4) (4m^3+6n)(6n-4m^3)=(6n+4m^3)(6n-4m^3)=36n^2-16m^6
5) 7(n^2-2)-4(3+n)(n-3)=7n^2-14-4(n+3)(n-3)=7n^2-14-4(n^2-9)=7n^2-14-4n^2+36=3n^2+22
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Answers & Comments
1) (3x+4)(3x-4)=(3x)^2-4^2=9x^2-16
2) (0,5b-6c)(0,5b+6c)=0,5^2×b^2-36c^2=0,25b^2-36c^2
3) (7x^2-y)(y+7x^2)=(7x^2-y)(7x^2+y)=49x^4-y^2
4) (4m^3+6n)(6n-4m^3)=(6n+4m^3)(6n-4m^3)=36n^2-16m^6
5) 7(n^2-2)-4(3+n)(n-3)=7n^2-14-4(n+3)(n-3)=7n^2-14-4(n^2-9)=7n^2-14-4n^2+36=3n^2+22