14A

15B

16B

17C

18B

19B

20C

\(16.\\ \left\{{}\begin{matrix}P+N+E=76\\P=E\\\left(P+E\right)-N=20\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}2P+N=76\\2P-N=20\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}P=E=Z=24\\N=28\end{matrix}\right.\)

\(20.\\ \left\{{}\begin{matrix}P+E+Z=40\\\left(P+E\right)-N=12\\P=E\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}2P+N=40\\2P-N=12\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}P=E=Z=13\\N=14\end{matrix}\right.\\ \Rightarrow A=Z+N=13+14=27\left(đ.v.C\right)\)

\(19\\ \left\{{}\begin{matrix}P+E+N=40\\P=E\\\dfrac{N}{P+E}=\dfrac{7}{13}\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}2P+N=40\\\dfrac{N}{2P}=\dfrac{7}{13}\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}2P+N=40\\13N-14P=0\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}P=E=Z=13\\N=14\end{matrix}\right.\\ \Rightarrow A=Z+N=13+14=27\left(đ.v.C\right)\\ \Rightarrow KH:^{27}_{13}R\)

=> Chọn B

\(1,\\ a,=6x^4y^4-x^3y^3+\dfrac{1}{2}x^4y^2\\ b,=4x^3+5x^2-8x^2-10x+12x+15\\ =4x^3-3x^2+2x+15\\ 2,\\ a,=7\left(x^2-6x+9\right)=7\left(x-3\right)^2\\ b,=\left(x-y\right)^2-36=\left(x-y-6\right)\left(x-y+6\right)\\ 3,\\ \Leftrightarrow x\left(x^2-0,36\right)=0\\ \Leftrightarrow x\left(x-0,6\right)\left(x+0,6\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x=0,6\\x=-0,6\end{matrix}\right.\)

A = 1/(5.6) + 1/(6.7) + ... + 1/(24.25)

= 1/5 - 1/6 + 1/6 - 1/7 + ... + 1/24 - 1/25

= 1/5 - 1/25

= 4/25

B = 2/(1.3) + 2/(3.5) + 2/(5.7) + ... + 2/(99.101)

= 1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + ... + 1/99 - 1/101

= 1 - 1/101

= 100/101

`a) A = 1/(5.6) + 1/(6.7)+...+1/(24.25)`

`= 1/5 - 1/6 + 1/6 - 1/7 +...+1/24-1/25`

`= 1/5-1/25`

`= 5/25 - 1/25`

`= 4/25`

Vậy:`A = 4/25`

`b) B = 2/(1.3)+2/(3.5)+...+2/(99.101)`

`= 1- 1/3 + 1/3 - .... +1/99-1/101`

`= 1 - 1/101`

`= 100/101`

Vậy: `B = 100/101`