Question

s=1*2+2*3+3*4.......+39*40

Answer 1

S= 1*2 + 2*3 + 3*4 +.....+ 39*40

S= [ 1*2*(3-0) + 2*3*(4-1) + 3*4*(5-2) +.....+ 39*40*(41-38) ] : 3

S= [ 1*2*3 - 0*1*2 + 2*3*4 - 1*2*3 + 3*4*5 - 2*3*4 +.....+ 39*40*41 - 38*39*40 ] : 3

S= 39*40*41 : 3

S= 21320

Answer 2

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