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3b=8c ⇒b=8c/3
teorema cosinusului
cos A=(b²+c²-a²)/2bc=[(8c/3)²+c²-(7c/3)²]/(2×8c/3×c)=
=(64c²/9+c²-49c²/9)/(16c²/3)=
=(15c²/9+c²)/(16c²/3)=
=24c²/9 ×3/16c²=
=8c²/3 ×3/16c²=1/2
sin A=√(1-cos²A)=√[1-(1/2)²]=√(1-1/4)=√3/4=√3/2
tg A/2=(1-cosA)/sinA=(1-1/2)/(√3/2)=1/2×2/√3=1/√3=√3/3⇒
m(∡A/2)=30° ⇒ m(∡A)=60°