d = <math><mi>d</mi><mo>=</mo><msqrt><msup><mfenced><mrow><msub><mi>x</mi><mn>2</mn></msub><mo>-</mo><msub><mi>x</mi><mn>1</mn></msub></mrow></mfenced><mn>2</mn></msup><mo> </mo><mo>+</mo><mo> </mo><msup><mfenced><mrow><msub><mi>y</mi><mn>2</mn></msub><mo> </mo><mo>-</mo><mo> </mo><msub><mi>y</mi><mn>1</mn></msub></mrow></mfenced><mn>2</mn></msup></msqrt></math> is called