A:Where was it
B:What was it
C:How was it
D:Why was it
已知向量 a→ =(2cos2x, 3 ), b→ =(1,sin2x),函数f(x)= a→ • b→ ﹣1.
清代恽寿平在《南田论画》中说:“元人幽亭秀木,自在化工之外,一种灵气。惟其品若天际冥鸿,故出笔便如衰弦急管,声情并集,非大地欢乐场中可得而拟议则也。”材料说明元朝时 ( )