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# If a circle...

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If a circle is centered at the origin and tangent to the line y=-x+4 , what is the area of the circle devided by pi ?

A) 2

B) 4

C) 8

D) 12

E) 16

Solveit  Dec 26, 2015

#2
+78719
+10

Notice that a tangent line drawn from the center of the circle to the point of tangency will meet the given line at right angles..........thus, these two lines will have reciprocal slopes....

And the equation of the line through the origin will be

y = x

So......setting these two equations equal, we have

x = -x + 4      add x to both sides

2x = 4

x = 2

So, at the point of tangency, y = x, so  y = 2

And the radius of such a circle centered at the origin will be

sqrt (2^2 + 2^2) = sqrt (8)

So....the area of this circle will be

A = pi [sqrt(8)]^2   =  8pi     and dividing this by pi   will produce a result of 8

Here's a graph of the situation ......https://www.desmos.com/calculator/g8eurlqsyo

CPhill  Dec 26, 2015
edited by CPhill  Dec 26, 2015
Sort:

#1
+2493
0
Solveit  Dec 26, 2015
edited by Solveit  Dec 26, 2015
edited by Solveit  Dec 26, 2015
edited by Solveit  Dec 26, 2015
edited by Solveit  Dec 26, 2015
#2
+78719
+10

Notice that a tangent line drawn from the center of the circle to the point of tangency will meet the given line at right angles..........thus, these two lines will have reciprocal slopes....

And the equation of the line through the origin will be

y = x

So......setting these two equations equal, we have

x = -x + 4      add x to both sides

2x = 4

x = 2

So, at the point of tangency, y = x, so  y = 2

And the radius of such a circle centered at the origin will be

sqrt (2^2 + 2^2) = sqrt (8)

So....the area of this circle will be

A = pi [sqrt(8)]^2   =  8pi     and dividing this by pi   will produce a result of 8

Here's a graph of the situation ......https://www.desmos.com/calculator/g8eurlqsyo

CPhill  Dec 26, 2015
edited by CPhill  Dec 26, 2015
#3
+2493
0

thanks CPhill ! :)

Solveit  Dec 26, 2015

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