题目

—When _________ your birthday? —_________ on December 2nd. A . is, It's B . is, It on C . are; They're D . does; On 答案：A已知函数f（x）=asinx+cosx-1的最大值是0．（1）求证：a=0；（2）若f（x+π4）=-13，求sin2x的值．