Đặt tổng trên là A ta có :
A= 1/ 20 + 1/ 30 + 1/ 42 + 1/ 56 + 1/ 72 + 1/90 + 1/110 + 1 / 123 + 1/ 156
= 1 / 4 x5 + 1/ 5 x 6 + 1/6x 7 + 1/ 7x8 + 1/8x9 + 1/9x10+ 1/ 10x11+ 1 /11x12 +1/12 x 13
= 1/4- 1/5 + 1/ 5 - 1/6 + 1/ 6 - 1/7 + 1/ 7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10+ 1/10 - 1/11 + 1/11 - 1/12+ 1/ 12 - 1/13
= 1 /4 - 1 /13
= 9 /52
A = 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/ 110 + 1/132 + 1/156
B=1/30+1/42+1/56+1/72+1/90+1/110+1/132+1/156
b = 1/30 + 1/42+ 1/56+1/72+1/90+1/110+1/132+1/156
tính nhanh
1/20+1/30+1/42+1/56+1/72+1/90+1/110+1/132
Tính nhanh:
1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 + 1/132
Tính nhanh
\(\dfrac{1}{20}\)+\(\dfrac{1}{30}\)+\(\dfrac{1}{42}\)+\(\dfrac{1}{56}\)+\(\dfrac{1}{72}\)+\(\dfrac{1}{90}\)+\(\dfrac{1}{110}\)+\(\dfrac{1}{132}\)
=
1/12+1/20+1/30+1/42+1/56+1/72+1/90+1/110+1/132
Giải ra cho mình nhé
1/30+1/42+1/56+1/72+1/90+1/110+1/132=21/x
tính nhanh nếu có thể
a 168 x 168 - 168 x 58 / 110
b 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 + 1/132 + 1/156
c 1/1/3 x 1/1/8 x 1/1/15 x 1/1/24 x 1/135 x ... x 1/1/99
cau C là hỗn số nha các bạn
