∠1＝45°,
tan∠2＝1/2,
tan∠3=1/3,
tan(∠2+∠3)=(1/2+1/3(1-1/2×1/3)=1,
∠2,∠3是锐角,
∠2+∠3＝45°,
∠1＋∠2+∠3＝90°.