Lecture 5 [02 March, 2020] MATH 127 (Prof Farid)

 MATH -127 (Coordinate Geometry)
Chapter-2 
(Pair of Straight Lines)



Question 6: Find the angle between the lines joining the origin to the points of intersection of the line  y3x=3y3x=3 with  x2+2xy+3y+4x+8y11=0x2+2xy+3y+4x+8y11=0  

Solution:   Given that,  y3x=3 
an
x2+2xy+3y+4x+8y11=0                

From equation (6.1) we get,
 y3x2=1        Here,  h=1,   a=7,   b=1                    

Now we can write, 
 x2+2xy+3y2+(4x+8y)(y3x2)11(y3x2)2=07x22xyy2=0     


                   

Let, θ be the angle between the lines represented by (6.3), then
  tanθ=2h2aba+b=21+771=286=223  


Chapter -3
The Circle


    ♦ Definition: A circle is the locus (set of points) of a point that moves so that the distance of a given point is always equal to a given distance. The given point is the center of the circle and the given distance is its radius.

    Standard form of the equation of a circle:  (xh)2+(yk)2=a2  
where (h,k) is the center and a = radius [a0]

        • Case 1: If a=0, equation (1) represents a point circle.
        • Case 2: If (0,0) is the center and radius = 1, then  x2+y2=1 represents a unit circle.
        • Case 3:  x2+y2=a2  

    ♦ General Form:  x2+y2+2gx+2fy+c=0(x2+2gx+g2)+(y2+2fy+f2)=g2+f2cNow,  Completing the square property, (x+g)2+(y+f)2=(g2+g2c)2  

   

Comparing it to the standard equation,
        center (g,f) and radius = g2+f2c

    ♦ Polar form of a circle :
•Pole on fixed point or initial point.
•Polar on fixed line or initial line.

  r122r1r2cos(θ1θ)=a2  

        • Case 1: if the pole lies on the circle,
then, r1=a, In this case  r=2a cos(θ1θ)  
        • Case 2: If θ=0 the initial line is the diameter and in this case, we have  r=2a cosθ1
        • Case 3: if x=rcosθ and y=rsinθ then  x2+y2=a2r2=a2r=a  



Upon which condition a point lies outside or inside a circle: 
  radius =  g2+f2c          center =  (g,f)     if, d>R(outside).     if, d=R(on the circle)     if, d<R(inside)  

Now, general forms of circle are:  x2+y2=a2     and     x2+y2+2gx+2fy+c=0  
    

        (1) The equation of the tangent line to the circle x2+y2=a2 at (x1,y1)is,  xx1+yy1=a2
        (2) The equation of the tangent line to the circle  x2+y2+2gx+2fy+c=0 at (x1,y1) is,  xx1+yy1+g(x+x1)+f(y+y1)+c=0  

♦ Some points to know:
    • Pole is a point, Polar is the line.
    • Pole may be inside the circle or outside the circle.
        If the pole is outside, polar will pass through the circle.
        If the pole is inside, polar will be outside the circle.

                     





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