1.

Find `(dy)/(dx)`, when : If `y=((cos x - sin x))/((cos x + sin x))`, prove that `(dy)/(dx)+y^(2)+1=0`.

Answer» `y((1-tanx)/(1+tanx))" [on dividing num. and denom. by cos x]"`
`rArry=tan((pi)/(4)-x)`
`rArr(dy)/(dx)=-sec^(2)((pi)/(4)-x)`
`rArr(dy)/(dx)+y^(2)+1=-sec^(2)((pi)/(4)-x)+tan^(2)((pi)/(4)-x)+1=0`.


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