... ó trng -Lp 4A, 4B, 5A, 5B, 5C -Lp 4A trng c 35 cõy, lp 4B trng c 28 cõy, lp 5A trng c 45 cõy, lp 5B trng c 40 cõy, lp 5C trng c 23 cõy -Khi lp cú lp tham gia trng cõy, ú l 5A, 5B, 5C -Cú lp trng ... l: biu thc tớnh s trung bỡnh cng l (36+ 42 + 57 ): = 45 c, khụng bt buc vit cõu tr li c S TB cng ca 34, 43 , 52 v 39 l: ( 34+ 43 + 52 + 39): = 42 ỏp s: Bi Hc sinh c yờu cu + Bi toỏn cho bit gỡ ? -S ... cng l (32 + 48 + 64 + 72) : = 54 - Hs c yờu cu -3 HS lờn bng lm bi, HS c lp lm vo VBT a S TB cng ca 42 v 52 l: (42 + 52 ): = 47 -GV cha bi Lu ý HS ch cn vit b S TB cng ca 36, 42 v 57 l: biu thc tớnh...