With the, resistance is always without propeller. That is why I said R dash. So, this I can call that equal to delta t. That is that augment of thrust that must be reduced from T to get R. So, T minus delta T is equal to R, delta T or delta R. Let me write delta T. So, you can understand T minus delta T equal to R. We can define a quantity small T which is equal to delta T by T which is equal to T minus R by T 1 minus R by T. This is called, this T is called thrust deduction fraction.