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Elementary Mathematics
If \( \Large x+y=7 \), then the value of \( \Large x^{3}+y^{3}+21xy \) is
A) 243
B) 143
C) 343
D) 443
Correct Answer:
C) 343
Description for Correct answer:
\( \Large x+y=7 \) (cubing both sides)
\( \Large \left(x+y\right)^{3}= \left(7\right)^{3} \)
\( \Large x^{3}+y^{3}+3 \left(x+y\right)xy = 343 \)
\( \Large x^{3}+y^{3}+21xy = 343 \)
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