Toán Lớp 6: Tính nhanh:1 x 2 x 3 x … x 19 x 20
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Toán Lớp 6: Tính nhanh:1 x 2 x 3 x … x 19 x 20
Comments ( 1 )
⇒A×3=1×2×3+2×3×3+3×3×4+…..+3×19×20
=1×2×3−0×1×2+2×3×4−1×2×3+3×4×5−2×3×4+…..+19×20×21−18×19×20
=19×20×21
=7980