(1+2+3)^2 =36

Yes, that's correct.

• Compute inside: 1 + 2 + 3 = 6
• Square: 6^2 = 36

If you expand (a + b + c)^2, you get a^2 + b^2 + c^2 + 2ab + 2bc + 2ca. With a=1, b=2, c=3: 1^2 + 2^2 + 3^2 + 2(1·2 + 2·3 + 3·1) = 1 + 4 + 9 + 2(2 + 6 + 3) = 14 + 22 = 36.

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