Toán Lớp 5: 1/20+1/44+1/77+1/119+1/170 Giúp mình với

0 bình luận về “Toán Lớp 5: 1/20+1/44+1/77+1/119+1/170 Giúp mình với”

1. Gọi 1/20 + 1/44 + 1/77 + 1/119 + 1/170 là A

   => 2 × A = 2/40 + 2/88 + 2/154 + 2/238 + 2/340

   => 2 × A = 2/{5 × 8} + 2/{8 × 11} + 2/{11 × 14} + 2/{14 × 17} + 2/{17 × 20}

   => 2/3 : A = 3/{5 × 8} + 3/{8 × 11} + 3/{11 × 14} + 3/{14 × 17} + 3/{17 × 20}

   => 2/3 : A = 1/5 – 1/8 + 1/8 – 1/11 + 1/11 – 1/14 + 1/14 – 1/17 + 1/17 – 1/20

   => 2/3 : A = 1/5 – 1/20

   => 2/3 : A = 3/20

   => A = 3/20 × 2/3

   => A = 1/10

   Vậy 1/20 + 1/44 + 1/77 + 1/119 + 1/170 = 1/10

   #dtkc

    

2. 1/20 + 1/44 + 1/77 + 1/119 + 1/170

    = 2/40 + 2/88 + 2/154 +  2/238 +  2/340

   = 2/(5xx8) + 2/(8xx11) + 2/(11xx14) + 2/( 14 xx 17) + 2/(17xx20)

   = 2/3 . ( 3/(5xx8) + 3/(8xx11) + 3/(11xx14) + 3/( 14 xx 17) + 3/(17xx20))

   = 2/3 . ( 1/5 – 1/8 + 1/8 – 1/11 + 1/11 – 1/14 + 1/14 – 1/17 + 1/17 – 1/20)

   = 2/3 . ( 1/5 – 1/20)

   = 2/3 . 3/20

   = 1/10