Answer :
Method 1.
Use [tex](a-b)(a+b)=a^2-b^2[/tex]
[tex](x-9)(x+9)=x^2-9^2=x^2-81[/tex]
Method 2.
Use distributive property: (a + b)(c + d) = ac + ad + bc + bd
[tex](x-9)(x+9)=(x)(x)+(x)(9)+(-9)(x)+(-9)(9)=x^2+9x-9x-81=x^2-81[/tex]
Method 1.
Use [tex](a-b)(a+b)=a^2-b^2[/tex]
[tex](x-9)(x+9)=x^2-9^2=x^2-81[/tex]
Method 2.
Use distributive property: (a + b)(c + d) = ac + ad + bc + bd
[tex](x-9)(x+9)=(x)(x)+(x)(9)+(-9)(x)+(-9)(9)=x^2+9x-9x-81=x^2-81[/tex]