giải giúp mik các bài này nha, nộp gấp .(giải thiks các bước lun nha) Bài 20 : thực hiện phép tính hợp lí a) (27-7).(-4)+31.(-12-8) b)135+(-241)-135+141 c)(43-13).(-9)+21.(-16-14) d)-(153-170-915)+(15+153-170) Bài 21: tính tổng(giải thik đầy đủ nha) a) A= 1-2+3-4+5-6+....+25-26 b) B=1-3+5-7+....+49-51 c) C= -1+3-5+7-...-53+55 d) D= 1+2-3+4+...+2019-2020 e) E = 1-6+11-16+21-26+....+2019-2020 f) F=2-5+8-11+...+110-113

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