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Q.Find the equation of a plane passing through the points −2i^+6j^−6k^-2\hat{i}+6\hat{j}-6\hat{k}, −3i^+10j^−9k^-3\hat{i}+10\hat{j}-9\hat{k} and −5i^−6j^−6k^-5\hat{i}-6\hat{j}-6\hat{k}.

Uttar Pradesh UpmspUP Board (UPMSP) Intermediate 2020Subjective· 5mImportance★★★★★
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Take two edge vectors from the points, cross them for a normal (−12,3,8)(-12,3,8), and write the plane through AA: 12x−3y−8z=612x-3y-8z=6.

Concept. A plane through three points has normal n⃗=AB⃗×AC⃗\vec n=\vec{AB}\times\vec{AC}; the equation is n⃗⋅(r⃗−A⃗)=0\vec n\cdot(\vec r-\vec A)=0.

The position vectors give points A(−2,6,−6)A(-2,6,-6), B(−3,10,−9)B(-3,10,-9), C(−5,−6,−6)C(-5,-6,-6).

Step 1 — edge vectors.

AB⃗=(−1,4,−3),AC⃗=(−3,−12,0).\vec{AB}=(-1,4,-3),\qquad \vec{AC}=(-3,-12,0).

Step 2 — normal (cross product).

n⃗=AB⃗×AC⃗=∣i^j^k^−14−3−3−120∣=i^(0−36)−j^(0−9)+k^(12+12)=(−36, 9, 24).\vec n=\vec{AB}\times\vec{AC}=\begin{vmatrix}\hat i&\hat j&\hat k\\-1&4&-3\\-3&-12&0\end{vmatrix}=\hat i(0-36)-\hat j(0-9)+\hat k(12+12)=(-36,\,9,\,24). …

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