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C1 Question

Really stuck with this 1 mark question ...

A circle with centre C (-5,6) touches the y axis as shown in the diagram

Find the equation of the circle in the form
(xa)2+(yb)2=r2(x-a)^2 + (y-b)^2 = r^2

i found that out ... (x+5)2+(y6)2=5(x+5)^2 + (y-6)^2 = 5,

BUT the next question says verify that the point P (-2,2) lies on the circle

i dont know how to work it out help pl0x
Original post by Skaterkid

Original post by Skaterkid
Really stuck with this 1 mark question ...

A circle with centre C (-5,6) touches the y axis as shown in the diagram

Find the equation of the circle in the form
(xa)2+(yb)2=r2(x-a)^2 + (y-b)^2 = r^2

i found that out ... (x+5)2+(y6)2=5(x+5)^2 + (y-6)^2 = 5,

BUT the next question says verify that the point P (-2,2) lies on the circle

i dont know how to work it out help pl0x


Sub x = -2 and y = 2 into the equation. It lies on the circle if the equation equals 5.
Reply 2
Original post by im so academic
Sub x = -2 and y = 2 into the equation. It lies on the circle if the equation equals 5.


I expanded the brackets and then subed it LHS is -16 and RHS is -20
Original post by Skaterkid

Original post by Skaterkid
I expanded the brackets and then subed it LHS is -16 and RHS is -20


Is the original equation correct?
Original post by Skaterkid


i found that out ... (x+5)2+(y6)2=5(x+5)^2 + (y-6)^2 = 5,





5 !!!!!! :frown:
Yes, remember the equation of the circle equals r squared. There is a mistake when you tried to find out the equation of the circle.
Original post by Skaterkid

Original post by Skaterkid
I expanded the brackets and then subed it LHS is -16 and RHS is -20


The RHS doesn't equal 20. You've substituted your values in wrong.
Original post by ghostwalker
5 !!!!!! :frown:

Yeah, I was a bit suspicious of that first. Usually the radius squared is square number.
(-2+5)^2=9
(2-6)^2=16
=25
Original post by iPthreefifthteen

Original post by iPthreefifthteen
(-2+5)^2=9
(2-6)^2=16
=25


:yep:
Reply 10
Original post by iPthreefifthteen
(-2+5)^2=9
(2-6)^2=16
=25


Is that the correct method to use in the exams?
Reply 11
Original post by Skaterkid
Is that the correct method to use in the exams?


It's as good as any :smile:

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