tana = sina+cosa = √2sin(a+π/4) ∵ 0＜a＜π/2 ∴ π/4＜a+π/4＜3π/4 ∴ 1＜√2sin(a+π/4)≤√2 ∴ 1＜tana≤√2 ∴ π/4＜a≤arctan√2 <π/3
