1/1*2 +1/2*3 +....+1/19*20=1-1/1*2+1/1*2-1/2*3 +......+1/18*19-1/19*20=
=1-1/19*20=379/380
1/1*2=1- 1/1*2
1/2*3=1/1*2-1/2*3
...........................
1/19*20=1/18*19 -1/19*20
14Х-(12Х-6+11Х)+2Х=Х+13-8Х
14Х-12Х+6-11Х+2Х=Х+13-8Х
2Х+2Х-11Х+7Х=+13-6
11Х-11Х=7
0Х≠7!