\( \mathop {\lim }\limits_{(x,y) \to ({\text{1,2)}}} (3x + 4y) = \)______
举一反三
- \( \mathop {\lim }\limits_{(x,y)______
- \(\mathop {\lim }\limits_{x \to 0} 2 { { \tan x - \sin x} \over { { {\sin }^3}x}}{\rm{ = }}\)______。______
- \(\lim \limits_{(x,y) \to (2,2)} { { x + y} \over {xy}} = \)______
- \( \lim \limits_{x \to 0} { { 9\sin x} \over { { x^3} + 3x}} = \)______ 。
- \( \mathop {\lim }\limits_{x \to 0} { { \left( { { e^x} - 1} \right)\sin 2x} \over {1 - \cos x}} = \)______ 。