The square root of -8 times the square root of -40

The answer is:  " -8 √5 " .

Note:


√-8 *√-40 = i√8 * i√40 ;  {since: "i" refers to the imaginary number; " √-1 " } ;

= i² * √8 *√40 ;


Note: 

"√8 * √40 = ? " ;

↔  " √8 = √4 √2 = 2√2 " ;

      "√40 = √4 √10 = 2√10" ;

→ "√8 √40 = 2 * 2 * √2 * √10 ;

                  = 4 * √(2*10)  ;

                  = 4 √20 ; 

 →  " √20 = √4 √5 = 2√5 ; 

 → 4 √20 = 4 * 2 * √5  

= 8 √5 ;

 → i² * 8 √5 ;

↔ 8 i² √5 ;

Note:  " i² = -1" ; 

→ 8(-1) * √5 ; 

= -8 √5 .

The correct answer is addition