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  1. If (a + b)2 = 100 and (a – b) = 4, then ab equals to :
    1. 116
    2. 84
    3. 21
    4. 53
Correct Option: C

4ab = (a + b)2 – (a – b)2
⇒  4ab = 100 – (4)2 = 100 – 16
⇒  4ab = 84

⇒  ab =
84
 = 21
4


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