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Lời giải:
$1+2+3+....+25$
$=(1+25)+(2+24)+(3+23)+(4+22)+(5+21)+(6+20)+(7+19)+(8+18)+(9+17)+(10+16)+(11+15)+(12+14)+13$
$=\underbrace{26+26+26+...+26}_{12}+13$
$=26\times 12+13=325$
a) \(A=\frac{27\text{x}45+27\text{x}25}{2+4+6+8+...+18}=\frac{27\left(45+25\right)}{\left(18+2\right)\text{x}\left(\frac{18-2}{2}+1\right)}\)
\(A=\frac{27\text{x}70}{20\text{x}9}=\frac{3\text{x}9\text{x}10\text{x}7}{10\text{x}2\text{x}9}\)
\(A=\frac{21}{2}\)
b) \(B=\frac{28\text{x}108-52\text{x}12}{32-28+24-20+16-12+8-4}\)
\(B=\frac{28\text{x}12\text{x}9\text{}\text{}-52\text{x}12}{\left(32-28\right)+\left(24-20\right)+\left(16-12\right)+\left(8-4\right)}\)
\(B=\frac{12\left(28\text{x}9-52\right)}{4+4+4+4}=\frac{12\text{x}200}{4\text{x}4}\)
\(B=\frac{4\text{x}3\text{x}4\text{x}50}{4\text{x}4}=150\)
\(\frac{48x700-24x45x20}{45-40+35-30+25-20+15-10}\)
\(=\frac{24x2x700-24x45x20}{5+5+5+5}\)
\(=\frac{24x1400-24x900}{20}\)
\(=\frac{24x\left(1400-900\right)}{20}\)
\(=\frac{24x500}{20}\)
\(=\frac{12000}{20}\)
\(=600\)
k nha
\(\frac{27x45+27x55}{2+4+6+...+16+18}\)
\(=\frac{27x\left(45+55\right)}{2+4+6+...+16+18}\)
\(=\frac{27x100}{2+4+6+...+16+18}\)
\(=\frac{2700}{\left(2+18\right)x9:2}\)
\(=\frac{2700}{90}\)
\(=30\)
k nha