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Worked solution

Sketch three upper solution relations and qualify their endpoints

Show the general solution of y dy/dx=2x−1y\,dy/dx=2x-1, y≥0y\ge0, is y2=2x2−2x+Cy^2=2x^2-2x+C. On one diagram sketch the members with C=−1,0,1C=-1,0,1.

CJC · Lecture · Example 13

Integrate the relation

∫y dy=∫(2x−1) dx\int y\,dy=\int(2x-1)\,dx gives y2/2=x2−x+Ay^2/2=x^2-x+A, hence y2=2x2−2x+Cy^2=2x^2-2x+C with C=2A. Since y is nonnegative, use the upper square root where real.

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Retain all three source members and calculator entries

The source enters Y1=2x2−2xY_1=\sqrt{2x^2-2x} (C=0), Y2=2x2−2x−1Y_2=\sqrt{2x^2-2x-1} (C=−1), Y3=2x2−2x+1Y_3=\sqrt{2x^2-2x+1} (C=1). For C=−1 the geometric x-intercepts are (1±3)/2≈−0.366,1.37(1\pm\sqrt3)/2\approx-0.366,1.37. For C=0 they are 0 and 1. The C=1 curve is smooth for all real x, with minimum (1/2,1/2)≈(0.5,0.707)(1/2,1/\sqrt2)\approx(0.5,0.707) and y-intercept 1.

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Distinguish the plotted closure from a differentiable solution

For C=−1 and C=0, the zero-height endpoints are valid points of the implicit squared relation, but not classical differentiable solutions of the original equation there: at y=0, the left side would be 0 while 2x−1 is nonzero. The classical solution branches therefore use the open intervals beyond those endpoints; the displayed figure marks them open while retaining their limiting coordinates. The Teacher note labels family sketches as outside the stated examination syllabus; this exercise is retained as source enrichment rather than removed.

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Compare your sketch

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Three upper curves on common axes: C=1 has minimum (1/2,1/sqrt2) and y-intercept 1; C=0 has open limiting endpoints x=0,1; C=−1 has open limiting endpoints (1−sqrt3)/2 and (1+sqrt3)/2, labelled approximately −0.366 and 1.37. All are symmetric about x=1/2; colors are supplemented by C and source Y labels. Open endpoints are excluded from classical differentiable solutions.
Three upper curves on common axes: C=1 has minimum (1/2,1/sqrt2) and y-intercept 1; C=0 has open limiting endpoints x=0,1; C=−1 has open limiting endpoints (1−sqrt3)/2 and (1+sqrt3)/2, labelled approximately −0.366 and 1.37. All are symmetric about x=1/2; colors are supplemented by C and source Y labels. Open endpoints are excluded from classical differentiable solutions.
Three upper curves on common axes: C=1 has minimum (1/2,1/sqrt2) and y-intercept 1; C=0 has open limiting endpoints x=0,1; C=−1 has open limiting endpoints (1−sqrt3)/2 and (1+sqrt3)/2, labelled approximately −0.366 and 1.37. All are symmetric about x=1/2; colors are supplemented by C and source Y labels. Open endpoints are excluded from classical differentiable solutions.

Common mistakes

A plotted endpoint of an implicit relation need not admit a finite derivative satisfying the original equation.

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