Practicing Success
If $a^2 + b^2 + c^2+216 = 12(a + b-2c)$, then $\sqrt{ab-bc + ca}$ is: |
8 3 6 4 |
6 |
(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 $\sqrt{ab-bc + ca}$ = $\sqrt{6 × 6 - 6 × -12 + (-12) × 6}$ $\sqrt{ab-bc + ca}$ = $\sqrt{36}$ = 6 |