Write down the curve and its general parametric point
xy=36,P(6t, t6) The parametric form generates every point of the conic as t varies.
Write the tangent at the point with parameter p
x+4y−24=0 Use the standard tangent formula.
Write the tangent at the point with parameter q
x+y−12=0 Use the standard tangent formula again.
Solve the pair of equations simultaneously
x=8,y=4 Eliminate one variable and back-substitute.
Substitute the given parameter value
Work with the specific point requested by the question.
Write down the coordinates of the point of contact
P(12, 3) Substitute the parameter into the parametric form.
Confirm the point lies on the curve
12×3=36 Both sides agree, so the point really is on the conic.
State the gradient of the tangent at this point
dxdy=−41 This comes from differentiating the curve implicitly.
State the gradient of the normal at this point
The normal gradient is the negative reciprocal of the tangent gradient.
Record the value of the constant in the curve
The constant fixes the size of the conic.
Recall the standard parabola and its parametric point
y2=4ax,P(at2, 2at) Every point of the parabola can be written in this parametric form.
Recall the focus and directrix of y2=4ax
S(a, 0),x=−a The focus is on the axis of symmetry and the directrix is the matching vertical line.
Recall the standard tangent to the parabola
ty=x+at2 This is the tangent at the point with parameter t.
Recall the standard normal to the parabola
y+tx=at3+2at This is the normal at the point with parameter t.
Select the matching option
R(8, 4) This is the point of intersection.