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( a + b ) ( c + d ) = a ( c + d ) + b ( c + d ) = ac + ad + bc + bd
→ → → → ↓ ↓ ↓ 即 ( a + b ) ( c + d ) = ac + ad + bc + bd ↑ ↑ ↑ → → → → → →
不知道這樣打是否有加深您的印象呢 ~?
參考資料: 我自己親手算 + 看國 2 數學課本參考
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