a) x + 7 = -12

x = (-12) - 7

x = 19

b) x - 15 = -21

x = (-21) + 15

x = -6

c) 13 - x = 20

x = 13 - 20

x = -7

4/1 x 13/15

= 52/15

52/15

mình có câu trl rồi nhé:

4 x 13/15 = 4 x 13/15 = 52/15 nhé!

=>4x=-20

hay x=-5

\(4x+15=-5\)

\(=> 4x=-20\)

\(=>x=-5\)

Vậy \(x=-5\)

\(4x+15=-5.\Leftrightarrow4x=-5-15.\Leftrightarrow4x=-20.\Leftrightarrow x=-5.\)

Chọn A

A

A

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

Câu 58: B

Câu 59: C