B

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MÌNH THẤY CÂU 35 BẠN LÀM RỒI MÀ

câu 35

bạn lam đúng rồi đó.

cái biểu đồ đó là thuộc nước singabo mà singabo lại thuộc môi trường xích đạo ẩm nên là môi trường xích đạo ẩm

Lời giải:

\(\lim\limits_{x\to 2}\frac{x^2+ax+b}{2x^2-x-6}=\lim\limits_{x\to 2}\frac{x^2+ax+b}{(x-2)(2x+3)}\)

Để giới hạn này là hữu hạn thì $x^2+ax+b\vdots x-2$

$\Rightarrow 2^2+a.2+b=0\Leftrightarrow 2a+b=-4$

Đáp án A.

\(2x^2-x-6=0\) có 1 nghiệm \(x=2\)

Do đó giới hạn đã cho là hữu hạn khi và chỉ khi \(x^2+ax+b=0\) cũng có 1 nghiệm \(x=2\)

\(\Rightarrow4+2a+b=0\Rightarrow b=-2a-4\)

Vậy:

\(\lim\limits_{x\rightarrow2}\dfrac{x^2+ax-2a-4}{2x^2-x-6}=\lim\limits_{x\rightarrow2}\dfrac{\left(x-2\right)\left(x+2\right)+a\left(x-2\right)}{\left(x-2\right)\left(2x+3\right)}\)

\(=\lim\limits_{x\rightarrow2}\dfrac{\left(x-2\right)\left(x+a+2\right)}{\left(x-2\right)\left(2x+3\right)}=\lim\limits_{x\rightarrow2}\dfrac{x+a+2}{2x+3}=\dfrac{a+4}{7}\)

\(\Rightarrow\dfrac{a+4}{7}=\dfrac{3}{2}\Rightarrow a=\dfrac{13}{2}\Rightarrow b=-2a-4=-17\)

\(\Rightarrow2a+b=-4\)

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\(5^3\cdot35+4^3\cdot7\)

\(=125\cdot35+64\cdot7\)

\(=4375+448=4823\)