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Senior one mathematics requires two people to teach the first question of group B on page A 144.
The center M(a, -2a) can be set from the meaning of the question.

And then r? = Horse? =(a-2)？ +(-2a+ 1)？ =5a？ -8a+5

If the circle is tangent to the straight line, the distance from the center of the circle M(a, -2a) to the straight line is d = r.

Distance formula from point to straight line, d=|-a- 1|/√2.

d？ =r？

Namely: (a+ 1)? /2=5a？ -8a+5

Answer? +2a+ 1= 10a？ - 16a+ 10

9a？ - 18a+9=0

Answer? -2a+ 1=0

a= 1

So, M( 1, -2), r? =2

So, the equation of a circle is: (x- 1)? +(y+2)？ =2

Have fun! I hope I can help you. If you don't understand, please ask. I wish you progress in your study! O(∩_∩)O

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