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计算:1×2+2×3+3×4+4×5+5×6+6×7+7×8+8×9=______.
人气:216 ℃ 时间:2019-12-24 21:26:03
解答
1×2+2×3+3×4+4×5+5×6+6×7+7×8+8×9
=2×(1+3)+4×(3+5)+6×(5+7)+8×(7+9)
=2×2×2+4×4×2+6×6×2+8×8×2
=4×2+16×2+36×2+64×2
=2×(4+16+36+64)
=2×120
=240
故答案为:240.
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