29.(2020•江苏四模)设HHH为ΔABC\Delta ABCΔABC的垂心(三角形三条高的交点),且3HA→+4HB→+5HC→=0→3\overrightarrow{HA}+4\overrightarrow{HB}+5\overrightarrow{HC}=\overrightarrow{0}3HA+4HB+5HC=0,则cos∠AHB\cos \angle AHBcos∠AHB的值为 ___−66-\frac{\sqrt{6}}{6}−66___.