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  1. If x + y = 7, then the value of x3 + y3 + 21xy is
    1. 243
    2. 143
    3. 343
    4. 443
Correct Option: C

Using Rule 8,
Given, x + y = 7
Now, x3 + y3 + 21xy = (x + y)3 – 3xy(x + y) + 21xy
= (7)3 – 3xy(7) + 21xy
= 343 – 21xy + 21xy = 343


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