Several methods were used to find log5 49 ×log7 125. Matthew Poulton of Queen Mary's Grammar School, Walsall changed the base to base 10 and someone, only identifying him or herself as Labboid, used a change of base to natural logarithms.


log5 49
= ln49 / ln5
log7 125
= ln125 / ln7.

Hence


log5 49 ×log7 125

=  ln49

ln5
×  ln125

ln1

=  ln(72) ×ln(53)

ln5 ×ln7

=  2ln7 ×3ln5

ln5 ×ln7

= 2 ×3 = 6

Patrick Snow, Saul Forresta, Hyeyoun Chung, Marc Carlambe and Arun Iyer, used this identity to find the solution as follows. Now log5 49=2log5 7 and log7 125 = 3log7 5 therefore

log5 49 ×log7 125 = 2log5 7 ×3log7 5 = 6×log5 7 ×log7 5 = 6.

Yatir Halevi generalized this problem to: loga bc ×logb ad. Using the logarithms rules: loga br = r×loga b and logb a = 1/loga b we get the expression equal to cd (loga b ×logb a) = cd. So our expression is equal to 2×3=6.