π/4＜α＜3π/4
cosα=5/13
sinα=根号(1-cos^2α)=12/13
0＜β＜π/4
sinβ=5/13
cosβ=根号(1-sin^2β)=12/13
sin(α+β)=sinαcosβ+cosαcosβ
= 12/13 * 12/13 + 5/13 * 5/13
= 1
π/4
sina=√(1-cos^2a)=12/13
0
cosb=√(1-sin^2b)=12/13
sin(a+b)=sinacosb+cosasinb
=12/13*12/13+5/13*5/13
=144/169+25/169
=169/169
=1
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