gwd-3-20滔滔

20: In the triangle above, is x > 90?(x是a,b之间的角)

(1)
              a² + b² < 15

(2)   c > 4 

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

D. EACH statement ALONE is sufficient.

E. Statements (1) and (2) TOGETHER are NOT sufficient.

只记得a²+b²=c^{2可以推知x=90}

^{什么时候学过}a²+b²<c^{2 推知x>90}

a²+b²>c^{2 推知x<90}

16<c2=a2+b2-2abcosx<15-2abcosx  美其名曰：余弦定理

=>-2abcosx>1   

=>cosx<0  

=>x>90度

以上是数学推理

画图也很easy啊，我推崇画图！可是我不知道怎么在这上面画，又懒得贴

就是a2+b2=c2 你先划一个直角三角形，但是这时候的c2<15对不？所以根据条件，c实际应该延长，所以所对的角就是钝角阿

牛啊