Because of 5x? +4y？ = 10x，

So y? =( 10x-5x？ )/4≥0.

The solution is 0≤x≤2.

x？ +y？ =x？ +( 10x-5x？ )/4=-x？ /4+5x/2。

The opening is downward, and the axis of symmetry is x=5, which increases monotonically in.

So when x=2, the maximum value is 4.

Solution 2: