theorem :: INTEGR10:7

for f being PartFunc of REAL,REAL
for a, b being Real st a < b & ['a,b'] c= dom f & f is_left_ext_Riemann_integrable_on a,b holds
for r being Real holds
( r (#) f is_left_ext_Riemann_integrable_on a,b & ext_left_integral ((r (#) f),a,b) = r * (ext_left_integral (f,a,b)) )