由20m²x²+11mnx-3n²=0
x²+11nx/20m-3n²/20m²=0
x²+11nx/20m+121n²/1600m²=3n²/20m²+121n²/1600m²
（x+11n/40m)²=361n²/1600m²
①x+11n/40m=19n/40m
即x=n/5m.
②x+11n/40m=-19n/40m
即x=-3n/4m.