# Thread: 2 calculus qestion

1. ## 2 calculus qestion

qestion 1
If fsubx(xsub0, ysub0) and fy(xsub0, ysub0) both exist, then f is continuous at (xsub0, ysub0). Prove or disprove

Hint consider the function defined by f(x,y) = (xy)/(x^2+y^2) , if (x,y) is not equal (0,0) or f(x,y) =0.

question 2
Let f(x,y) = (sin^2 (x-y))/ (abs(x) +abs(y) ).
Prove that lim as (x,y) approach (0,0) of f(x,y)=0.

Hint: for all real numbers m and n. abs(sin(m+n)) smaller than or equal to abo(m+n) smaller than or equal to abs(m) +abs(n).

note: abs(m) means absolution value of m.

Thank you very much

2. Originally Posted by littlemu
qestion 1
If fsubx(xsub0, ysub0) and fy(xsub0, ysub0) both exist, then f is continuous at (xsub0, ysub0). Prove or disprove

Hint consider the function defined by f(x,y) = (xy)/(x^2+y^2) , if (x,y) is not equal (0,0) or f(x,y) =0.
False. Use the example the book gives.

The function f(x,y) is defined on an open disk containing (0,0). Which means f is continous if and only if,
lim [(x,y)-->(0,0)] f(x,y) = f(0,0)=0

But if you choose the path x=y both approaching zero the limit of f(x,y) is then 1! It must always be zero if it continous.

This is Mine 55th Post!!!

3. I shall use my own inequality which is simple but useful.

Let x,y>=0 then sqrt(x+y) <= sqrt(x)+sqrt(y)