√x³-x²y+1/4y² - √1/4x³-x²y+xy²
=1/2√4x³-4x²y+y² -1/2 √x(x²-4xy+4y²)
=1/2√4x²(x-y)+y² -1/2 √x(x-2y)²
=1/2√4*25(25-15)+15² -1/2(2*15-25) √25
=1/2√1225 -25/2
=35/2-25/2
=5