Segitiga ABC mempunyai panjang sisi AB =3 cm, BC=7cm dan AC=5cm besar sudut A adalah A.30 B.60 C.90 D.120 E.150 Bantu jawab pakay cara

[tex]
\cos{A}\\
=\frac{AB^2+AC^2-BC^2}{2\cdot AB\cdot AC}\\
=\frac{3^2+5^2-7^2}{2\cdot 3\cdot 5}\\
=\frac{9+25-49}{30}\\
=-\frac{15}{30}\\
=-\frac{1}{2}\\
\text{Jadi }A=\cos^{-1}{\left(-\frac{1}{2}\right)}=180\degree-60\degree=120\degree
[/tex]

Cos A=AB²+AC²-BC²/2.AB.AC
Cos A=9+25-49/2.3.5
Cos A=-15/30
Cos A=-1/2
A=120°