(a) A normal to the plane is n=(2,−1,2), with ∣n∣2=9. At C, the signed plane expression is 2(2)−(−1)+2(3)−9=2. The foot of the perpendicular is H=C−92n=(914,−97,923). This is the centre of the circle.
(b) The centre-to-plane distance is d=2/3. Using the right triangle formed by the sphere radius and circle radius ρ, ρ2+d2=25⇒ρ=25−94=3221.
Determines the normal and signed plane expression.
[1 mark]
Determines the foot and centre of the circle.
[2 marks]
Determines the centre-to-plane distance.
[1 mark]
Uses the Pythagorean radius relation.
[1 mark]
Obtains the exact radius.
[1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.