\(1\cdot5+2\cdot6+\cdots+41\cdot45\)

\(=1\cdot\left(1+4\right)+2\cdot\left(2+4\right)+\cdots+41\cdot\left(41+4\right)\)

\(=\left(1^2+2^2+\cdots+41^2\right)+4\left(1+2+\cdots+41\right)\)

\(=\frac{41\left(41+1\right)\left(2\cdot41+1\right)}{6}+4\cdot\frac{41\cdot42}{2}\)

\(=\frac{41\cdot42\cdot83}{6}+2\cdot41\cdot42=41\cdot7\cdot83+2\cdot41\cdot42=27265\)

  1x5+2x6+3x7+4x8+...+50x54

=(1+2+3+4+...50)x(5+6+7+78+...+54)

=1275x1269=1617975

\(A=\frac{4}{3X7}+\frac{4}{7X11}+\frac{4}{11X15}+...+\frac{4}{100X104}\)

\(=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+...+\frac{1}{100}-\frac{1}{104}\)

\(=\frac{1}{3}-\frac{1}{104}\)

\(=\frac{101}{312}\)

Chúc bạn học giỏi nha!!!

K cho mik với nhé nguyen huu thuong 2005

\(A=\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+...+\frac{4}{100.104}\)

\(A=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+...+\frac{1}{100}-\frac{1}{104}\)

\(A=\frac{1}{3}-\frac{1}{104}=\frac{104}{312}-\frac{3}{312}=\frac{101}{312}\)

1.5+1.5

=5+5

=10

=10

các bạn cho mình vài li-ke cho tròn 545 với 

1.5+1.5

=5+5

=10

a.

\(3x^7+x^4-3x^7+x^5+x+4=x^5+x^4+x+4\)

Đa thức có bậc 5

b.

Đa thức có bậc 0