we see now in aviation world many passenger airplanes flying at around 35000feet where the density and mass is very low, in order to avoid drag and have less fuel consumption.but how come the engine produce thrust with less amount of air intake as the density is very low at high altitude?
The trust does reduce with higher altitude. However, this is somewhat compensated by the plane going faster (equivalent airspeed) relative to the ground. The decreasing density of air which reduces both the maximum thrust available and the maximum lift is what controls the maximum altitude an airplane can fly.
The engines produce less thrust, but there is also a lot less air resistance, so they are actually able to fly faster at high altitude, because the air resistance diminishes faster than the engine thrust. That's one reason why high altitudes are preferred for airline flights. It's faster and it uses less fuel.
