the answer:
the main rule:
for all value positive of a real b, √b²= b, and √b*c= √b * √c, if c is a real positive
let A=√25x^4, we know that 25x^4=5² * x² * x²
therefore,
√25x^4= √5² * x² * x² =√5² √x² √x² = 5* x*x= 5x²
the final answer is A=√25x^4=5x²