Porisma

Worked solution

Find the unique line–plane crossing

Find the point of intersection of the line

r=(4,−6,−2)+λ(1,−2,0),λ∈R,\mathbf r=(4,-6,-2)+\lambda(1,-2,0),\quad\lambda\in\mathbb R,

and the plane r⋅(2,−1,2)=4\mathbf r\cdot(2,-1,2)=4.

CJC · Lecture · Example 31

Answer

B=(52,−3,−2)B=(\frac52,-3,-2).

Compare your reasoning

Let the intersection be B. Since B lies on the line, its position is (4+λ,−6−2λ,−2)(4+\lambda,-6-2\lambda,-2). Since it also lies on the plane,

2(4+λ)−(−6−2λ)+2(−2)=4,8+2λ+6+2λ−4=4,10+4λ=4,λ=−32.\begin{aligned}2(4+\lambda)-(-6-2\lambda)+2(-2)&=4,\\8+2\lambda+6+2\lambda-4&=4,\\10+4\lambda&=4,\\\lambda&=-\frac32.\end{aligned}

Substitute into the line:

OB→=(4−32,−6+3,−2)=(52,−3,−2).\begin{aligned}\overrightarrow{OB}&=(4-\tfrac32,-6+3,-2)\\&=(\tfrac52,-3,-2).\end{aligned}

The direction–normal product is (1,−2,0)⋅(2,−1,2)=4≠0(1,-2,0)\cdot(2,-1,2)=4\ne0, so this is the unique crossing. As an independent check, the point gives 2(5/2)−(−3)+2(−2)=42(5/2)-(-3)+2(-2)=4. The source names the intersection B; that label is retained.

How did this question go?