Question

a+b=3 ,a-b= √5 , find the value of ab

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Answer 1

$ab = {(a + b) ^{2} - {(a - b)}^{2} } \div 4 \\ = {3}^{2} - { \sqrt{5} }^{2} \div 4 \\ = 9 - 5 \div 4 \\ = 4 \div 4 \\ = 1$