This is not difficult to explain why 1250 came.
( 100-2x)(50-2x)=3600
2(50-x)2(25-x)=3600 equation, extract 2 from each bracket on the left.
Equation (50-x)(25-x)=900 is divided by 4 at the same time, and the two extracted 2s are omitted on the left.
x？ -75x+ 1250=900 Take the left (50-x)25+(50-x)(-x) according to the above formula.
As for the final simplified result, it is obtained by subtracting 900 from both sides of the equal sign.
The second intermediate solution is obtained by directly removing the brackets with the above formula without extracting 2.
5000-200X- 100X+4X^2=3600
Then subtract 3600 at the same time.
4X^2-300X+ 1400=0
The final factorization is to use the above formula in reverse.
(4X-20)(X-70)=0
When two numbers are multiplied by zero, at least one number is zero.
Or 4x=20 x=5.
Or x-70=0 x=70.
You can ask me if you don't understand anything. I wish you progress in your study.