Points and Circles (Leaving Cert Mathematics): Flashcards

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Points and Circles
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What are the three possible positions of a point relative to a circle?

Inside, on, outside

Point inside circle: distance from centre vs radius

Distance < radius

Point on circle: distance from centre vs radius

Distance = radius

Point outside circle: distance from centre vs radius

Distance > radius

Distance formula between (x1,y1)(x_1, y_1) and (x2,y2)(x_2, y_2)

d=(x2−x1)2+(y2−y1)2d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}

Standard circle equation (centre at origin)

x2+y2=r2x^2 + y^2 = r^2

If x2+y2=kx^2 + y^2 = k, what is rr?

r=kr = \sqrt{k}

Substitution method: if x2+y2<r2x^2 + y^2 < r^2, point is...

Inside the circle

Substitution method: if x2+y2=r2x^2 + y^2 = r^2, point is...

On the circle

Substitution method: if x2+y2>r2x^2 + y^2 > r^2, point is...

Outside the circle

Fastest method for circles centred at origin

Substitution method

Method that works for all circle equations

Distance comparison method

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