3. 41^2-31^2 = (41-31) * (41+31) = 10 * 72 = 720
4. 37^2-23^2=(37-23)*(37+23)=14*60=840
5. 79^2-65^2\420=((79-65)(79+65))\420=(144*14)/420=2016/420=4,8
6. (63^2-27^2)/(83^2-79^2)=((63-27)(63+27))/((83-79)(83+39))=(36*90)/(4*122)=3240/488=1620/244=810/122=405/61