Question

tính nhanh

1/15+1/35+1/63+1/99+1/143

Answer 1

1/15 + 1/35 + 1/63 + 1/99 + 1/143

đặt A = 1/15 + 1/35 + 1/63 + 1/99 + 1/143

A       = 1/3X5 + 1/5X7 + 1/7X9 + 1/9X11 + 1/11X13

A x 2 = 2 x ( 1/3x5 + 1/5x7 + 1/7x9 + 1/9x11 + 1/11x13 )

A x 2 = 2/3x5 + 2/5x7 + 2/7x9 + 2/9x11 + 2/11x13

A x 2 = 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + 1/11 - 1/13

A x 2 = 1/3 - 1/13

A x 2 = 13/39 - 3/39

A x 2 = 10/39

A      =10/39 : 2

A       = 5/39

Answer 2

1/15 + 1/35 + 1/63 + 1/99 + 1/143   = 5/39 nhé bạn !

Answer 3

A=1/3*5+1/5*7+1/7*9+1/9*11+1/11*13

2A=2/3*5+2/5*7+2/7*9+2/9*11+2/11*13=2/3-2/5+2/5-2/7+2/7+2/9-2/9+2/11-2/11+2/13=2/3+2/13=26/39+6/39        =32/39

=>A=16/39