4x - 3y -6z = 0 .(1)
x + 2y - 7z = 0 .(2)
4 * (2) - (1)
11y - 22z = 0 => y = 2z
带入(2)式得到 x = 7z - 2y = 3z
所以所求的式子
= ( (3z)² + 5 * (2z)² + 7z²) / ( 2* 3z * 2z + 2* 2z * z - 6* 3z *z)
= [(9 + 20 + 7)z²] / [(12 + 4 - 18) z²]
= 36 / -2
= -18