Question

已知a=(根号7)-1,求代数式3(a^3)+12(a^2-)6a-12

Answer 1

3(a^3)+12(a^2-)6a-12

=3[a^3+4a^2-2a]-12

=3a[(a+2)^2-6]-12

=3a(a+2)^2-18a-12

原式=3[(根号7）-1][(根号7）+1][(根号7）+1]-18[(根号7）-1]-12

=18[(根号7）+1]-18[(根号7）-1]-12

=36-12

=24