Question

In the session 32 there is differintiation (33:27)
\sqrt { (a-x)^{ 2 }+(b-y)^{ 2 } }
Professor wrote on the board:
\frac { (a-x)+(b-y)\frac { dy }{ dt } }{ \sqrt { { { (a-x }) }^{ 2 }+{ (b-y) }^{ 2 } } }
But what i get after solving this:
\sqrt { (a-x)^{ 2 }+(b-y)^{ 2 } } =((a-x)^{ 2 }+(b-y)^{ 2 })^{ \frac { 1 }{ 2 } }=\frac { 1 }{ 2 } w^{ -\frac { 1 }{ 2 } }(-2a+2x-2b\frac { dy }{ dt } +2y\frac { dy }{ dt } )=\frac { (a-x)+(b-y)\frac { dy }{ dt } }{ \sqrt { { { (a-x }) }^{ 2 }+{ (b-y) }^{ 2 } } }
Where is the error?