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2、说明：^——表示次方
(a+3b)^7
T4=T(3+1)
=C(7,3)a^(7-3)(3b)^3
=(7×6×5)/(3×2×1)a^4×27b^3
=35×27a^4b^3
=945a^4b^3
a^2b^5中r=5
T(5+1)=C(7,5)a^(7-5)(3b)^5
=C(7,2)a^2×243b^5
=(7×6)/(2×1)×243a^2b^5
=21×243a^2b^5
=5103a^2b^5
(a+3b)^7
T4=T(3+1)
=C(7,3)a^(7-3)(3b)^3
=(7×6×5)/(3×2×1)a^4×27b^3
=35×27a^4b^3
=945a^4b^3
a^2b^5中r=5
T(5+1)=C(7,5)a^(7-5)(3b)^5
=C(7,2)a^2×243b^5
=(7×6)/(2×1)×243a^2b^5
=21×243a^2b^5
=5103a^2b^5
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