Basic loggin' 2

Algebra Level 3

Given that log 2 x = 3 \log _{ 2 }{ x } =3 , find the value of the expression below:

A = 1 log 3 2 × log 4 3 × log 5 4 × log 6 5 × log 7 6 × log x 7 A=\frac { 1 }{ \log _{ 3 }{ 2 } \times \log _{ 4 }{ 3 } \times \log _{ 5 }{ 4 } \times \log _{ 6 }{ 5 } \times \log _{ 7 }{ 6 } \times \log _{ x }{ 7 } }


The answer is 3.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Ponhvoan Srey
Oct 10, 2015

A = 1 log 3 2 × log 4 3 × log 5 4 × log 6 5 × log 7 6 × log x 7 F r o m log 2 x = 3 , w e o b t a i n x = 8. P l u g x = 8 i n t o A : A = 1 log 3 2 × log 4 3 × log 5 4 × log 6 5 × log 7 6 × log 8 7 O b s e r v e t h a t log b a × log c b = log b a × 1 log b c = log b a log b c = log c a . A s f o r o u r p r o b l e m : log 7 6 × log 8 7 = log 8 6 log 6 5 × log 8 6 = log 8 5 log 5 4 × log 8 5 = log 8 4 T h i s g o e s o n u n t i l w e a r r i v e a t : A = 1 log 3 2 × log 8 3 = 1 log 8 2 = 1 1 3 S o t h e a n s w e r i s A = 3 . \\ A=\frac { 1 }{ \log _{ 3 }{ 2 } \times \log _{ 4 }{ 3 } \times \log _{ 5 }{ 4 } \times \log _{ 6 }{ 5 } \times \log _{ 7 }{ 6 } \times \log _{ x }{ 7 } } \\ \\ From\quad \log _{ 2 }{ x } =3,\quad we\quad obtain\quad x=8.\\ \\ Plug\quad x=8\quad into\quad A:\\ A=\frac { 1 }{ \log _{ 3 }{ 2 } \times \log _{ 4 }{ 3 } \times \log _{ 5 }{ 4 } \times \log _{ 6 }{ 5 } \times \log _{ 7 }{ 6 } \times \log _{ 8 }{ 7 } } \\ \\ Observe\quad that\quad \log _{ b }{ a } \times \log _{ c }{ b=\log _{ b }{ a } \times \frac { 1 }{ \log _{ b }{ c } } = } \frac { \log _{ b }{ a } }{ \log _{ b }{ c } } =\log _{ c }{ a } .\\ As\quad for\quad our\quad problem:\quad \log _{ 7 }{ 6 } \times \log _{ 8 }{ 7 } =\log _{ 8 }{ 6 } \\ \qquad \qquad \qquad \qquad \qquad \qquad \log _{ 6 }{ 5 } \times \log _{ 8 }{ 6 } =\log _{ 8 }{ 5 } \\ \qquad \qquad \qquad \qquad \qquad \qquad \log _{ 5 }{ 4 } \times \log _{ 8 }{ 5 } =\log _{ 8 }{ 4 } \\ This\quad goes\quad on\quad until\quad we\quad arrive\quad at:\quad A=\frac { 1 }{ \log _{ 3 }{ 2 } \times \log _{ 8 }{ 3 } } =\frac { 1 }{ \log _{ 8 }{ 2 } } =\frac { 1 }{ \frac { 1 }{ 3 } } \\ So\quad the\quad answer\quad is\quad \boxed { A=3 } .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...