Răspuns:
Explicație pas cu pas:
1/1*2 = 1/1 - 1/2
1/2*3 = 1/2 - 1/3
1/3*4 = 1/3 - 1/4
1/4*5 = 1/4 - 1/5
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1/1*2 + 1/2*3 + 1/3*4 + 1/4*5 = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5
= 1/1 - 1/5 = 5/5 - 1/5 = 4/5
1/1*2 + 1/2*3 + 1/3*4 + 1/4*5 = 4/5 adevarat
