解:y=sina+cosa=(√2)sin[a+(π/4)].因角a为⊿的内角,故0π/4-(√2)/2-1<(√2)sin[a+(π/4)]≤√2.===>-1
(sinA+sinB)2=sinA2+sinB2+2sinAsinB∵sinA2+sinB2=1 sinAsinB>0∴(sinA+sinB)2>1 sinA+sinB>1注:后面的2是平方