\(\frac53+\frac{5}{15}+\frac{5}{35}+\ldots+\frac{5}{399}\)

\(=\frac{5}{1\cdot3}+\frac{5}{3\cdot5}+\frac{5}{5\cdot7}+\cdots+\frac{5}{19\cdot21}\)

\(=\frac52\cdot\left(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}_{}+\cdots+\frac{2}{19\cdot21}\right)\)

\(=\frac52\cdot\left(\frac11-\frac13+\frac13-\frac15+\cdots+\frac{1}{19}-\frac{1}{21}\right)\)

\(=\frac52\cdot\left(1-\frac{1}{21}\right)=\frac52\cdot\frac{20}{21}=\frac{50}{21}\)

Noãn

3

củ hành

\(x^2-4x+4-y^2=\left(x-2\right)^2-y^2=\left(x+y-2\right)\left(x-y-2\right)\)

đề là tìm GTLN hay là GTNN bạn

B

Có thể vì bạn chưa đủ 50 xu

1+65=66

345+98765=99110

123456678-123456678=0

\(B=4+4^2+4^3+\cdots+4^{100}\)

\(=\left(4+4^2\right)+\left(4^3+4^4\right)+\cdots+\left(4^{99}+4^{100}\right)\)

\(=4\left(1+4\right)+4^3\left(1+4\right)+\cdots+4^{99}\left(1+4\right)\)

\(=5\left(4+4^3+4^5+\cdots+4^{99}\right)\) CHIA HẾT CHO 5

VẬY B CHIA HẾT CHO 5