(20) Assume that the function value of at A(x, y) has a relative maximum. A(x, y)=?

(19) Let ,

(18) Find the equation of the tangent plane to the surface of at the point (-2, 1, 8).

50. Put the paragraphs into the correct order, with the given topic sentence below.
[Topic sentence] The restaurant, which has just been opened in my neighborhood, provides a great dining experience.
A The customers can enjoy the meals with a variety of music there.
B In addition, there are a variety of live band performances every Friday evening.
C The featured cuisines really make the restaurant excel.
D They are prepared by a well-known chef to delight every customer's taste buds.
E At first, I thought it would be just like a café where people could go for a quick meal, but it turns out to be much more than that.
(A) CEBDA
(B) CBDAE
(C) EABCD
(D) ECDBA

49. Put the paragraphs into the correct order, with the given topic sentence below.
[Topic sentence] Although online courses have advantages, face-to-face courses are still essential for several reasons.
A One of the most important reasons is that face-to-face courses encourage more social interaction.
B For example, in some science courses, it is a must for students to conduct experiments in the laboratories.
C This helps students develop interpersonal communication skills which are vital for the real job market.
D To sum up, although online learning is a recent trend for education, it still cannot replace face-to-face courses.
E Another reason is that some courses require hands-on experiences in a physical classroom.
(A) ABCED
(B) ACEBD
(C) BCAED
(D) BDEAC

48. Put the paragraphs into the correct order, with the given topic sentence below.
[Topic sentence] Both sprinters and marathoners are runners, but they are very different.
A The training sessions help them build fast and explosive muscles to accelerate actions.
B As the races only last for a few seconds, these runners use strength training to enhance their performance on a regular basis.
C Compared to sprinters, marathoners run for around 1,600 to 10,000 meters long.
D Sprinters run relatively short distances for around 100-400 meters in high speed.
E They typically have a thinner and leaner muscle tone which helps them perform better in long distance.
(A) DBACE
(B) DABEC
(C) CEDAB
(D) ECABD