Geometry · real student question

Find where the circle (x - 0.5334)^2 + (y - 0.5334)^2 = 0.3475^2 meets the line sqrt(3)/3 * x - y = 0.

Question

Find the intersection points of the circle

(x0.5334)2+(y0.5334)2=0.34752(x-0.5334)^2+(y-0.5334)^2=0.3475^2

and the line 33xy=0\tfrac{\sqrt3}{3}x-y=0.

Step-by-step solution

  1. Rewrite the line as yy in terms of xx.

    y=33x=x30.5773503xy=\frac{\sqrt3}{3}x=\frac{x}{\sqrt3}\approx 0.5773503\,x

    The line passes through the origin at an angle of 3030^\circ, since its slope is tan30\tan 30^\circ.

  2. Substitute into the circle equation. Replacing yy turns a two-variable system into a single quadratic:

    (x0.5334)2+(x30.5334)2=0.34752(x-0.5334)^2+\left(\frac{x}{\sqrt3}-0.5334\right)^2=0.3475^2

    Expanding and collecting powers of xx gives Ax2+Bx+C=0Ax^2+Bx+C=0 with

    A=1+13=43,B=2(0.5334)2(0.5334)3=1.6827173,C=2(0.5334)20.34752=0.4482749A=1+\frac13=\frac43,\qquad B=-2(0.5334)-\frac{2(0.5334)}{\sqrt3}=-1.6827173,\qquad C=2(0.5334)^2-0.3475^2=0.4482749

  3. Test the discriminant before solving.

    Δ=B24AC=2.83153712.3907990=0.4407381>0\Delta=B^2-4AC=2.8315371-2.3907990=0.4407381>0

    A positive discriminant means the line genuinely cuts the circle twice; zero would mean tangency and negative would mean the line misses it entirely. Checking this first tells you how many answers to expect.

  4. Solve the quadratic.

    x=1.6827173±0.44073812(4/3)=1.6827173±0.66388112.6666667x=\frac{1.6827173\pm\sqrt{0.4407381}}{2(4/3)}=\frac{1.6827173\pm 0.6638811}{2.6666667}

    x1=0.879974,x2=0.382064x_1=0.879974,\qquad x_2=0.382064

  5. Recover the yy-coordinates from the line.

    y1=0.5773503(0.879974)=0.508053,y2=0.5773503(0.382064)=0.220585y_1=0.5773503(0.879974)=0.508053,\qquad y_2=0.5773503(0.382064)=0.220585

    so the intersection points are (0.8800,0.5081)(0.8800,\,0.5081) and (0.3821,0.2206)(0.3821,\,0.2206).

  6. Verify both points lie on the circle. Substituting back, (0.8799740.5334)2+(0.5080530.5334)2=0.120756=0.34752  (0.879974-0.5334)^2+(0.508053-0.5334)^2=0.120756=0.3475^2\;\checkmark, and the same holds for the second point to twelve decimal places. Note that rounding the first point to (0.8806,0.5084)(0.8806,\,0.5084) would put it off the circle by about 4×1044\times 10^{-4}.

Answer

(0.8800,  0.5081)and(0.3821,  0.2206)(0.8800,\;0.5081)\quad\text{and}\quad(0.3821,\;0.2206)

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