Răspuns:
Explicație pas cu pas:
2/1*3 = 1 - 1/3
2/3*5 = 1/3 - 1/5
......
2/39*41 = 1/39 - 1/41
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2/1*3 + 2/3*5 + ...+ 2/39*41 = 1 - 1/3 + 1/3 - 1/5 + ... + 1/39 - 1/41
= 1 - 1/41 = 41/41 - 1/41 = 40/41
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3/1*4 = 1 - 1/4
3/4*7 = 1/4 - 1/7
......................
3/37*40 = 1/37 - 1/40
................................
3/1*4 + 3/4*7 + ...+ 3/37*40 = 1 - 1/4 + 1/4 - 1/7 + ....+ 1/37 - 1/40
= 1 - 1/40 = 40/40 = 1/40 = 39/40
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40/41 - 39/40 = 40*40/40*41 - 39*41/40*41 = (1600 - 1599)/40*41 = 1/40*41
_______________________
80*82/40*41 = 4
x = √4 = 2
