Question

20分求一道题!x:y:z=1;2;3,xy+yz+xz=66,求2X的平方+12Y的平方-9Z的平方?

Answer 1

x:y:z=1;2;3

所以y=2x,z=3x

y^2=4x^2

z^2=9x^2

xy+yz+xz=66

2x^2+6x^2+3x^2=66

x^2=6

2x^2+12y^2-9z^2

=2x^2+12*4x^2-9*9x^2

=-31x^2

=-186