Đáp án:
$\displaystyle \begin{array}{{>{\displaystyle}l}} 20072008\ \times \ 20202021\ =405495127128168\\ \end{array}$
Giải thích đáp án:
$\displaystyle \begin{array}{{>{\displaystyle}l}} Đặt\ A=20072008.20202021\\ Ta\ có:\\ 20072008=20000000+70000+2000+8\\ 20202021=20000000+200000+2000+20+1\\ Đặt\ 20000000=a;70000=b;2000=c;8=d,200000=e;20=f\\ \ và\ 1=g\\ \Rightarrow A=( 20000000+70000+2000+8) .\\ ( 20000000+200000+2000+20+1)\\ =a^{2} +ae+ac+af+ag\ +\ ba+be+bc+bf+bg\ \\ +\ ca+ce+c^{2} +cf+cg\ +\ da+de+dc+df+dg\\ =4.10^{14} +4.10^{12} +4.10^{10} +4.10^{8} +0,2.10^{6} \ \\ +1,4.10^{12} +1,4.10^{10} +1,4.10^{8} +1,4.10^{6} +7.10^{4}\\ +\ 4.10^{10} +4.10^{8} +4.10^{6} +4.10^{4} +0,2.10^{4}\\ +1,6.10^{8} +1,6.10^{6} +1,6.10^{4} +1,6.10^{2} +8\\ =4.10^{14} +5,4.10^{12} +9,4.10^{10} +11.10^{8} +7,2.10^{6} +12,8.10^{4} +1,6.10^{2} +8\\ =405495127128168 \end{array}$
