∠ACB=45°由题意 |OA|=|OB|=|OC|∵3OA+4OB=-5OC∴(3OA+4OB)²=(-5OC)²,即9|OA|²+16|OB|²+24OA*OB=25|OC|²∴OA*OB=0∴OA⊥OB,∠AOB=90°∴∠ACB=(1/2)∠AOB=45°(同弧所对圆周角是圆心角的一半)
