The answer is: " -8 √5 " .
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Note:
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√-8 *√-40 = i√8 * i√40 ; {since: "i" refers to the imaginary number; " √-1 " } ;
= i² * √8 *√40 ;
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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 .
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