[mathislifeyess]

ความจริงข้อนี้แทนsinx=a/c cotx=b/a ตรงตรงก็จบครับ
2(a^2/c^2)=2+(a/c)(b/a)
2(a^2/c^2)=2+(b/c)
\because (a^2)+(b^2)=(c^2)
2(1-(b^2/c^2))=2+(b/c)
2-2(b^2/c^2)=2+(b/c)
0=2(b^2/c^2)+(b/c)
\therefore (b/c)=0,-1/2
ซึ่ง b/c=cosx
\therefore x=90,120 but 0<x<=90
\therefore x=90