50 000 x 10 = 500 000 N
500 000/12 = drag = 41666.666 N
Thrust = 2 engines x 50 000N = 100 000 N
Now (Thrust - drag) / weight) x 100= climb gradient
((100 000 – 41666.666) / 500 000) x 100 = 11,666 %
The formula given in the notes is for small angles/gradients of climb
where lift is considered the same as weight (remember it is in fact less than
weight) in a balance climb forces along the flight path are equal to forces at
right angles to the flight path.
If you know the lift drag ratio and are given the mass you can find the
value of drag.