(62  +72 +82+92+102) - (12  +22 +32+42+52)=

(62 -12  ) +(72  - 22 )+( 82 - 32 )+( 92 - 42 )+(102  - 52 )=

(6 -1) x(6+1)+(7 - 2 )(7+2)+( 8 - 3 )(8+3)+( 9 - 4)(9+4)+(10  - 5 )(10+5)

= 5 (7+9+11+13+15)=5x55=275

=36+49+64+81+100-1-4-9-16-25

=275

a) \(\left(\dfrac{1}{4}\right)^3.\left(\dfrac{1}{8}\right)^2=\left(\dfrac{1}{2}\right)^6.\left(\dfrac{1}{2}\right)^6=\left(\dfrac{1}{2}\right)^{12}\)

b) \(4^2.32:2^3=2^4.4.8:2^3=2^4.2^2.2^3:2^3=2^6\)

c) \(5^2.5^3.\dfrac{1}{5^4}.5^3\)\(=5^5.\dfrac{1}{5^4}.5^3=\dfrac{5^5}{5^4}.5^3=5.5^3=5^4\)

d) \(5^6.\dfrac{1}{20}.2^2.3^3:125=5^6.\dfrac{1}{5}.\dfrac{1}{4}.2^2.3^3:5^3\)\(=\dfrac{5^6}{5}.\dfrac{2^2}{4}.3^3:5^3=5^5.3^3:5^3=5^2.3^3\)

151 và 33

66

Giá tiền `40` quyển vở :

`5400 xx 40 = 216000(đồng)`

Số tiền `12` cái bút chì :

`2500 xx 12 = 30000(đồng)`

Số tiền mua `24` cây bút chi :

`3228800 - 216000 - 30000 = 2982800(đồng)`

Số tiền mua `1` cây bút chì :

`2982800 : 24 = 372850/3(đồng)`

`=>` Đề sai

A= { x ϵ N /  x là các số lẻ < 21 }

\(a,225:15+3\left(2x+1\right)=270\\ 15+3\left(2x+1\right)=225:270\\ 15+3\left(2x+1\right)=\dfrac{5}{6}\\ 3\left(2x+1\right)=\dfrac{5}{6}:3\\ 2x+1=\dfrac{5}{18}\\ 2x=\dfrac{5}{18}-1\\ 2x=\dfrac{5-18}{18}\\ 2x=-\dfrac{13}{18}\\ x=\left(-\dfrac{13}{18}\right):2\\ x=-\dfrac{13}{36}\)

\(b,19.3-9\left(x-2\right)=0\\ 57-9\left(x-2\right)=0\\ 9\left(x-2\right)=57\\ x-2=57:9\\ x-2=\dfrac{19}{3}\\ x=\dfrac{19}{3}+2\\ x=\dfrac{19+6}{3}\\ x=\dfrac{25}{3}\)