A= 7/4 (33/12 +33/20+33/30+33/42)
A= 7/4 * 33 * (1/12+1/20+1/30+1/42)
A= 7/4 * 33 * (1/3*4 + 1/4*5 +1/5*6 +1/6*7)
A= 231/4 (1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7)
A=231/4(1/3-1/7)
A=231/4(7/21-3/21)
A=231/4 * 4/21
A=11
Vậy A=11
Ta có A=7/4.(3333/1212+3333/2020+3333/3030+3333/4242)
=7/4.(11/4+33/20+11/10+11/14
=7/4.44/7
=11