展開の公式（基礎編）
\[ (a-b)^2 = a^2 - 2ab + b^2 \]
\[ (a+b)(a-b) = a^2 - b^2 \]
\[ (x+a)(x+b) = x^2 + (a+b)x + ab \]
\[ (ax+b)(cx+d) = acx^2 + (ad+bc)x + bd \]
\[ (a+b+c)^2 = a^2 + b^2 + c^2 + 2ab + 2bc + 2ca \]
\[ (a+b)^2 = a^2+2ab+b^2 \]
\[ (a-b)^2 = a^2 - 2ab + b^2 \]
\[ (a+b)(a-b) = a^2 - b^2 \]
\[ (x+a)(x+b) = x^2 + (a+b)x + ab \]
\[ (ax+b)(cx+d) = acx^2 + (ad+bc)x + bd \]
\[ (a+b+c)^2 = a^2 + b^2 + c^2 + 2ab + 2bc + 2ca \]
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