a) -1≤x≤1
⇒-3≤3x≤3
⇒7≥4-3x≥1
b)0≤√cosx≤1
⇒0≤2√cosx≤2
⇒1≤2√cosx+1≤3
c)0≤√(1-sinx)≤√2
-1≤√(1-sinx)-1≤√(2)-1
-1≤y≤√2 -1
d)sinx+cosx=√2sin(x+π/4)
-1≤sin(x+π/4)≤1
⇒-√2≤√2sin(x+π/4)≤√2
⇒-√2≤y≤√2
e)sinx-cosx-2=√2sin(x-π/4)-2
-1≤sin(x-π/4)≤1
⇒-√2≤√2sin(x-π/4)≤√2
⇒-√2-2≤√2sin(x-π/4)-2≤√2-2
vì √2sin(x-π/4)-2 luôn ≥0 ∀x
mà √2 -2 <0
⇒ko tìm được x
⇒ko có gt max min