The answer is: 45 * 10 ⁻¹¹ .
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Step-by-step explanation:
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Given: (9 * 10⁻⁵) * (5 *10 ⁻⁶) ; Simplify.
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 (9 * 10⁻⁵) * (5 *10⁻⁶) = 
9 * 5 * 10⁻⁵ * 10⁻⁶ = 
(9 * 5) * 10⁻⁵ * 10⁻⁶ 
45 * 10⁻⁵ * 10⁻⁶ ; 
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Note the following property of exponents:
xᵃ * xᵇ = x⁽ᵃ⁺ᵇ⁾ ;
As such, 10⁻⁵ * 10⁻⁶ = 10⁽⁻⁵ ⁺ ⁻⁶) = 10 ⁻¹¹ ;
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So; 45 * 10⁻⁵ * 10⁻⁶ 
 = 45 * 10⁻¹¹ .
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