Question

(3a-2b的平方)+(3a+2b)(3a-2b)-9(a+b)的平方,其中a=-1/2,b=1/2

Answer 1

(3a-2b^2)+(3a+2b)(3a-2b)-9(a+b)^2

= (3a-2b^2)+(9a^2-4b^2)-9(a^2+2ab+b^2)

=3a-2b^2+9a-4b^2-9a^2-18ab+9b^2

=3a^2-18ab-15b^2

将a=-1/2,b=1/2代入上式中得,

3(-1/2)^2-18(-1/2)x(1/2)-15(1/2)^2

=3x(1/4)+18x(1/4)-15(1/4)

=3/2