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P = 5.6.7 + 6.7.8 + 7.8.9 + 20.21.22
4P = 5.6.7.(8-4) + 6.7.8.(9 - 5) + ...... + 20.21.22.(23-19)
4P = 5.6.7.8 - 4.5.6.7 + 6.7.8.9 - 5.6.7.8 + .... + 20.21.22.23 - 19.20.21.22
4P = 20.21.22.23 - 4.5.6.7 = 211680
P = 211680 / 4= 52920
4P=5.6.7.(8-4)+6.7.8.(9-5)+7.8.9.(10-6)+.......+20.21.22.(23-19)
4P=5.6.7.8-4.5.6.7+6.7.8.9-5.6.7.8.9+.............+20.21.22.23-19.20.21.22
4P=20.21.22.23
P=20.21.22.23:4
P=53130
P = 5.6.7+ 6.7.8 + 7.8.9 + ...+ 20.21.22 = 50736 nha bạn .
4P = 4.5.6.7+4.6.7.8+4.7.8.9+...+4.20.21.22
= 5.6.7.(8-4)+6.7.8.(9-5)+7.8.9.(10-6)+....+20.21.22.(23-19)
= 5.6.7.8-4.5.6.7+6.7.8.9-5.6.7.8+7.8.9.10-6.7.8.9+....+20.21.22-23-19.20.21.22
= 20.21.22.23-4.5.6.7 = 212520 - 840 = 211680
=> P = 52920
k mk nha
5*6*7+6*7*8+7*8*9+..............................+20*21*22
5+8+9+10++11+12+13+14+15+16+17+18+19+20+21+22=230
vậy p=230
P = 5.6.7+ 6.7.8 + 7.8.9 + .... + 20.21.22
4P = 5.6.7.(8-4) + 6.7.8.(9 - 5) + .... + 20.21.22.(23 - 19)
4P = 5.6.7.8 - 4.5.6.7 + 6.7.8.9 - 5.6.7.8 + 20.21.22.23-19.20.21.22
4P = 20.21.22.23 - 4.5.6.7 = 211680
P = 211680/4= 52920
a) B= 5.6.7+6.7.8+7.8.9+...+2015.2016.2017
4B=4.(5.6.7+6.7.8+7.8.9+...+2015.2016.2017)
4B=5.6.7.4+6.7.8.4+7.8.9.4+...+2015.2016.2017.4
4B=5.6.7.(8-4)+6.7.8.(9-5)+7.8.9.(10-6)+...+2015.2016.2017.(2018-2014)
4B=5.6.7.8-4.5.6.7+6.7.8.9-5.6.7.8+7.8.9.10-6.7.8.9+...+2015.2016.2017.2018-2014.2015.2016.2017
4B= -4.5.6.7
B=\(\dfrac{-4.5.6.7}{4}\)
B= -210
b) D=7.9.11+9.11.13+11.13.15+...+101.103.105
8D= 8(7.9.11+9.11.13+11.13.15+...+101.103.105)
8D=7.9.11.8+9.11.13.8+11.13.15.8+...+101.103.105.8
8D=7.9.11.(13-5)+9.11.13.(15-7)+11.13.15.(17-9)+...+101.103.105.(107-99)
8D=7.9.11.13-5.7.9.11+9.11.13.15-7.9.11.13+11.13.15.17-9.11.13.15+...+101.103.105.107-99.101.103.105
8D=-5.7.9.11
D= \(\dfrac{-5.7.9.11}{8}\)
D= \(\dfrac{3465}{8}\)
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