Which of these is the equation for a line tangent to the circle x2 + y2 = 20 at point (2, -4)?

A) y=1/2x+√20
B) y=2x+√20
C) y=2x-5
D) 1/2x-5

Question

Which of these is the equation for a line tangent to the circle x2 + y2 = 20 at point (2, -4)?

A) y=1/2x+√20
B) y=2x+√20
C) y=2x-5
D) 1/2x-5

tkhunny · Answer

You have the point.  Check to see that it's on the circle.
Find the slope, there.  You may need an implicit derivative.

mathmale · Answer

Remember:  the derivative of a function represents the rate of change at a given x.
Can you differentiate the relationship that describes this circle?

danjs · Answer

or find the slope of the radius to that point, and the tangent is perpendicular

danjs · Answer

slope radius from center to given point =   -4/2 = -2

the tangent is perpendicular to the radius for a point on the circle, the perpendicular slope to -2 is        +1/2

put the line together from the point and its slope..

anonymous · Answer

Thanks guys! I got it. :)