rút gọn rồi quy đồng mẫu: A=2002/2000.16-1970 B=1.2.3+2.4.6+4.8.12+7.14.21/1.3.6+2.6.12+4.12.24+7.21.42

0 bình luận về “rút gọn rồi quy đồng mẫu: A=2002/2000.16-1970 B=1.2.3+2.4.6+4.8.12+7.14.21/1.3.6+2.6.12+4.12.24+7.21.42”

1. $\begin{array}{l}\bullet\text{ Rút gọn}\\A=\dfrac{2002}{2000.16-1970}\\\to A=\dfrac{2002}{(2002-2).16-1970}\\\to A=\dfrac{2002}{2002.16-2.16-1970}\\\to A=\dfrac{2002}{2002.16-32-1970}\\\to A=\dfrac{2002}{2002.16-2002}\\\to A=\dfrac{2002}{2002.15}\\\to A=\dfrac1{15}\\\,\\B=\dfrac{1.2.3+2.4.6+4.8.12+7.14.21}{1.3.6+2.6.12+4.12.24+7.21.42}\\\to B=\dfrac{1.2.3+2.4.6+4.8.12+7.14.21}{1.2.3.3+2.4.6.3+4.8.12.3+7.14.21.3}\\\to B=\dfrac{1.2.3+2.4.6+4.8.12+7.14.21}{3.(1.2.3+2.4.6+4.8.12+7.14.21)}\\\to B=\dfrac13\\\,\\\bullet\text{ Quy đồng mẫu số :}\\\text{- Ta có : $\rm BCNN(15,3)=15$}\\\to\begin{cases} \dfrac1{15}=\dfrac{1.1}{15.1}=\dfrac{1}{15}\\\dfrac13=\dfrac{1.5}{3.5}=\dfrac5{15}\end{cases} \end{array}$

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