1, Chọn đáp án C.
Ta có: V= \(\dfrac{1}{3}\)S.h
\(\Rightarrow\) 126 = \(\dfrac{1}{3}\)S.6
\(\Rightarrow\) S = 126 : 2 = 63\(\left(cm^3\right)\)
2, Chọn đáp án D
ĐKXĐ: x≠1 ; x≠3
Khi đó : \(\dfrac{x-5}{x-1}+\dfrac{2}{x-3}=1\)
\(\Leftrightarrow\dfrac{\left(x-5\right)\left(x-3\right)}{\left(x-1\right)\left(x-3\right)}+\dfrac{2\left(x-1\right)}{\left(x-1\right)\left(x-3\right)}=\dfrac{\left(x-1\right)\left(x-3\right)}{\left(x-1\right)\left(x-3\right)}\)
\(\Rightarrow\) (x−5) (x−3) + 2 (x−1) = (x−1) (x−3)
\(\Leftrightarrow\) \(x^2-8x+15+2x-2=x^2-4x+3\)
\(\Leftrightarrow\) −8x + 2x + 4x = 3 − 15 + 2
\(\Leftrightarrow\) −2x = −10
\(\Leftrightarrow\) x = 5 ( thỏa mãn ĐKXĐ )
Vậy S = \(\left\{5\right\}\)
