câu1: vì a≥0, b≥0
⇒(√a ×√b)²= a×b=(√ab)²⇒đpcm
câu2: VP= (√8-5√2+√20)×√5 - (3√$\frac{1}{10}$+10)
=(2√2-5√2+2√5)√5 - (3√$\frac{1}{10}$+10)
= 10- 3√10 - 3√$\frac{1}{10}$-10
=-3√10 - 3√$\frac{1}{10}$
=-3 (√10+√$\frac{1}{10}$)= -3×$\frac{11×√10}{10}$ =-3,3×√10 =VT