y=cos²x+3cosx+2
=cos²x+3cosx+9/4-9/4+2
=(cosx+3/2)²-1/4
∵x∈[0,π/2）
∴cosx∈(0,1]
即cosx+3/2∈(3/2,5/2]
(cosx+3/2)²∈(9/4,25/4]
∴y∈(2,6]
即函数的值域为(2,6]
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