2025 SST PRELIMS P1 Q10

2025 SST PRELIMS P1 Q10

Secondary 4
8 marks

Points \(P(-6,-12)\) and \(Q(2,-20)\) lie on a circle \(C_1\) with centre \(A\).

The line \(7y+4x+87=0\) passes through the centre of the circle.

  1. Find the coordinates of \(A\) and the radius of the circle, \(C_1\).[5]
  2. Hence find the equation of the circle \(C_1\).[1]
  3. Another circle \(C_2\) is obtained by reflecting circle \(C_1\) in the line \(y=x\). Find the equation of the circle, \(C_2\).[2]

Solution:

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Answer:(i) \(A=(1,-13)\), \(r=5\sqrt{2}\) (ii) \((x-1)^2+(y+13)^2=50\) (iii) \((x+13)^2+(y-1)^2=50\)

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