Prove that the curves xy = 4 and x2 + y2 = 8 touch each other.

Given:


Curves xy = 4 ...(1)


& x2 + y2 = 8 ...(2)


Solving (1) & (2),we get


xy = 4


x


Substituting x in x2 + y2 = 8,we get,


()2 + y2 = 8


+ y2 = 8


16 + y4 = 8y2


y4 – 8y2 + 16 = 0


We will use factorization method to solve the above equation


y4 – 4y2 – 4y2 + 16 = 0


y2(y2 – 4) – 4(y2 – 4) = 0


(y2 – 4)(y2 – 4) = 0


y2 – 4 = 0


y2 = 4


y = ±2


Substituting y = ±2 in x,we get,


x


x = ±2


The point of intersection of two curves (2,2) &


( – 2, – 2)


First curve xy = 4


1×y + x. = 0


x. = – y


m1 ...(3)


Second curve is x2 + y2 = 8


Differentiating above w.r.t x,


2x + 2y. = 0


y. = – x


m2 ...(4)


At (2,2),we have,


m1



m1 = – 1


At (2,2),we have,


m2



m2 = – 1


Clearly, m1 = m2 = – 1 at (2,2)


So, given curve touch each other at (2,2)


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