cosa+sina-2
=√2(√2/2cosa+√2/2sina)-2
=√2sin(a+π/4)+2
因：-1≤sin(a+π/4)≤1
所以可得 ：-√2+2≤sin(a+π/4)+2≤√2+2
即：cosq+sina-2的最大值为：√2+2
注：sinπ/4=cosπ/4=√2/2