B = 3/1².2² + 5/2².3² + 7/3².4² + ... + 19/9².10²

B = 3/1.4 + 5.4.9 + 7/9.16 + ... + 19/81.100

B = 1 - 1/4 + 1/4 - 1/9 + 1/9 - 1/16 + ... + 1/81 - 1/100

B = 1 - 1/100 < 1 ( đpcm)

5*2=10

Đề đúng: \(\sqrt{2x+4+6\sqrt{2x-5}}+\sqrt{2x-4+2\sqrt{2x-5}}=4\)

ĐK: \(x\ge\frac{5}{2}\)

Pt trên tương đương: \(\sqrt{2x-5+6\sqrt{2x-5}+9}+\sqrt{2x-5+2\sqrt{2x-5}+1}=4\)

<=>\(\sqrt{\left(\sqrt{2x-5}+3\right)^2}+\sqrt{\left(\sqrt{2x-5}+1\right)^2}=4\)

<=>\(\sqrt{2x-5}+3+\sqrt{2x-5}+1=4\)

<=>\(2\sqrt{2x-5}=0\Leftrightarrow2x-5=0\Leftrightarrow x=\frac{5}{2}\left(\text{nhận}\right)\)

Vậy S={5/2}

a) A = { C ; O ; A ; L } 

b) B = { O ; N ; L ; I ; N ;E ; M ; A ; T ; H } 

c) { H;O;C;T;AN}

d) D  = { C;H;A;M;H;O;C}

e) E = { T;O;A;N} 

a)-55+(-13)

=-(55+13)

=-68

b)4+(-10)

=-(10-4)

=-6