y' = x² + y² - r² (r = 0, 1, 2, ..., 7)

y' = (x + y)/(x - y) (method 1)

y' = (x + y)/(x - y) (method 2)

y' = sin(x)/y + sin(y)/x

y' = cos(x) + cos(y)

y' = cos(x) sqrt(1+x²) + cos(y) sqrt(1+y²)

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( Mathematical software used: Graph )