1.

If a curve `y=f(x)`passes through the point `(1,-1)`and satisfies the differentialequation `,y(1+x y)dx""=x""dy`, then `f(-1/2)`is equal to:(1) `-2/5`(2) `-4/5`(3) `2/5`(4) `4/5`A. (a) -2/5B. (b) -4/5C. (c) 2/5D. (d) 4/5

Answer» Correct Answer - (d)
Given differential equation is
`y(1+xy)dx=x dy`
`rArr y dx+xy^(2)dx=x dy`
`rArr (x dy- y dx)/y^(2) =x dx`
`rArr -((x dy- y dx))/y^(2) =x dx rArr -d (x/y)=x dx`
On integrating both sides, we get
`-x/y=x^(2)/2+C …(i)`
`therefore` It passes through (1,-1).
`therefore 1=1/2+C rArr C=1/2`
Now, from Eq. (i) `-x/y=x^(2)/2+1/2`
` rArr x^(2)+1=-(2x)/y`
` rArr y=-(2x)/(x^(2)+1)`
`therefore f(-1/2)=4/5`


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