Question

If $9\times 3=36$;
$11\times 7=81$, then what is the value of $5\times 13$ ?

• A. 65
• B. 66
• C. 81
• D. 51
• E. 72

Answer 1

The correct option is C 81

The rule is : $a\times b={\left(\frac{a+b}{2}\right)}^2$
$\therefore 5\times 13={\left(\frac{5+13}{2}\right)}^2=9^2=81$