2x3+3x4+4x5+.+19x20=?

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2x3+3x4+4x5+.+19x20=?
2x3+3x4+4x5+.+19x20=?

2x3+3x4+4x5+.+19x20=?
先提供两个数列前n项和公式：
Sn = 1² + 2² + 3² + …… + n² = n(n+1)(2n+1)/6
sn = 1+2+3+……+n = n+n(n-1)/2
原式 = (3-1)×3 + (4-1)×4 + (5-1)×5 + …… + (20-1)×20
= 3²+4²+5²+……20² -(3+4+5+…… +20)
= S(20)-S(2) + s(20)-s(2)
= 20(20+1)(40+1)/6 -5 + 20+20*19/2 - 3
= 2870 -5 +20 +190 -3
= 3072

2x3+3x4+4x5+.......+19x20
= 2*2 + 2 + 3*3 + 3 + 4*4 + 4 + ... + 19*19 + 19
= 2^2 + 3^2 + ... + 19^2 + 2 + 3 + ... + 19
= (19)(20)(39)/6 - 1 + 20*10 - 1
=2470 - 2 +200
= 2668你们俩谁...

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2x3+3x4+4x5+.......+19x20
= 2*2 + 2 + 3*3 + 3 + 4*4 + 4 + ... + 19*19 + 19
= 2^2 + 3^2 + ... + 19^2 + 2 + 3 + ... + 19
= (19)(20)(39)/6 - 1 + 20*10 - 1
=2470 - 2 +200
= 2668

收起

2x3+3x4+4x5+.+19x20=? (2X2)/1X3+(4X4)/(3X5)+(6X6)/(5X7)+...+(20X20)/(19X21)==? 1x2/1+2x3/1+3x4/1+4x5/1.19x20/1 1/1X3+1/2X4+1/3X5+.+1/18X20=? 1/1x3+1/2x4+1/3x5.1/18x20=?简便算法, 1/1x3+1/2x4+1/3x5+……+1/18x20=? 1X3/1+2X4/1+3X5/1+.+18X20/1=? 1/2x3十1/3x4十1/4x5十……十1/18x19十1/19x20=? 计算：1/1X3+1/2X4+1/3X5+.+1/18X20=?并说明理由快加把劲 1/1x2+1/2x3+1/3x4+1/4x5+1/5x6+.+1/18x19+1/19x20 计算方法与结果数学简便计算-32x4/3x3/2=?10/9X6/5x5/4=?7/3x14x9/2=?18/25x15/4x20/9=? 1x3分之1+2x4分之1+3x5分之1+.+18x20分之1 1/x3+1/2x4+1/3x5+...+1/18x20要列式哟! 1x2+2x3+3x4+.+19x20 解方程组:x1+x2+x3+x4+x5=7 3x1+2x2+x3+x4-3x5=－2 x2+2x3+2x4+6x5=23 5x1+4x2-3x3+3x4-x5=10 x1-x2+x3=1 x2-x3+x4=2 x3-x4+x5=3 x4-x5+x1=4 x5-x1+x2=5 求x1,x2,x3,x4,x5 1x2＋2x3＋3x4……＋19x20=? 求非齐次线性方程组的通解x1+3x2+5x3-4x4=1x1+3x2+2x3-2x4+x5=-1x1-2x2+x3-x4-x5=3x1+2x2+x3-x4-x5=3