Turning to ， we completely ignore the information in 。 Although contains a lot of information， it also is not sufficient. For example， the following sequences each satisfy ， yet they have different first terms：

　　1， 3， 12

　　3， 9， 36

　　This eliminates B， and our chances of guessing correctly have increased to 1　in 2.

　　Next， we consider and together. From ， we know the 3rd term of S　is 4. From ， we know the 3rd term is four times the 2nd. This is equivalent to saying the 2nd term is 1/4 the 3rd term： 4 = 1. Further， from， we know the 2nd term is three times the 1st. This is equivalent to saying the 1st term is 1/3 the 2nd term： 1 = 1/3. Hence， the first term　of the sequence is fully determined： 1/3， 1， 4. The answer is C.

　　Example 2： In the figure to the right， what is the area of the triangle?　

　　Recall that a triangle is a right triangle if and only if the square of the　longest side is equal to the sum of the squares of the shorter sides 。 Hence， implies that the triangle is a right triangle. So　the area of the triangle is /2. Note， there is no need to calculate the areawe just need to know that the area can be calculated. Hence， the answer is either A or D.

　　Turning to ， we see immediately that we have a right triangle. Hence， again the area can be calculated. The answer is D.

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