Question

по формуле сокрощенного умножения​

Answer 1

$\displaystyle\bf\\69^{2} =(70-1)^{2} =70^{2} -2\cdot 70\cdot 1+1^{2} =4900-140+1=4761\\\\79\cdot 81=(80-1)\cdot(80+1)=80^{2} -1^{2}=6400-1= 6399\\\\42\cdot 38=(40+2)\cdot(40-2)=40^{2} -2^{2} =1600-4=1596\\\\101^{2} =(100+1)^{2} =100^{2}+2\cdot 100\cdot 1+1^{2} =10000+200+1=10201\\\\99^{2} =(100-1)^{2} =100^{2}-2\cdot 100\cdot 1+1^{2} =10000-200+1=9801\\\\105^{2} \cdot 95^{2}=(100+5)^{2} \cdot(100-5)^{2} =100^{2} -5^{2} =10000-25=9975$