Tính:
a. (x + 2y)2
b. (x – 3y)(x + 3y)
c. (5 – x)2
a. (x + 2y)2 = x2 + 4xy + 4y2
b. (x – 3y)(x + 3y) = x2 – (3y)2 = x2 – 9y2
c. (5 – x)2 = 52 – 10x + x2 = 25 – 10x + x2
-- Mod Toán 8
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