Practicing Success
If $a^2+b^2+c^2+216=12(a+b-2 c)$, then $\sqrt{a b-b c-c a}$ is: |
$3 \sqrt{5}$ $8 \sqrt{5}$ $6 \sqrt{5}$ $4 \sqrt{5}$ |
$6 \sqrt{5}$ |
(a + b)2 = a2 + b2 + 2ab (a – b)2 = a2 + b2 – 2ab a2 + b2 + c2 + 216 = 12(a + b – 2c) a2 + b2 + c2 + 216 = 12(a + b – 2c) = a2 – 12a + b2 – 12b + c2 + 24c + 216 = 0 = a2 – 12a + 36 + b2 – 12b + 36 + c2 + 24c + 144 = 0 = (a – 6)2 + (b – 6)2 + (c + 12)2 = 0 a = 6 b = 6 c = –12 $\sqrt{a b-b c-c a}$ = $\sqrt{6 × 6 - 6 × -12 - (-12) × 6}$ $\sqrt{a b-b c-c a}$ = $\sqrt{180}$ = $6 \sqrt{5}$ |