(x+a)^4 = ^4C_0 x^4 + ^4C_1 x^{4-1} a + ^4C_2x^{4-2}a^2 + ^4C_3x^{4-3}a^3 + ^4C_4x^{4-4}a^4
= x^4 + 4x^3a + 6x^2a^2 + 4xa^3 + a^4
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