dare to solve this logarithmic problem

log7 log7{7+[7+(7+.......∞)1/2]1/2}1/2

find its value.....

21 Answers

71
Vivek @ Born this Way ·

let the infinite term be y.then

log7log7y

taking log on an indefinite i.e.,∞ no. gives infinite no. so,

log7log7y = y

1
pritishmasti ............... ·

nihal what are the given options????

just solve ur self ...the equation is :

y = log7log7(Z)

z = \sqrt{z+7}

z^{2} = z + 7

z^{2}-z-7=0

z = \frac{1 +- \sqrt{1^{2} -(4*7*1)}}{2}

z = \frac{1 +-\sqrt{29}}{2}

z = 1 - √292 is discarded as it is less than 1

so z = 1 + √292

1
nihal raj ·

acc to pritishmast ans should be -0.265 but it is not in the options means that method is wrong as cal. are correct.....

qwerty can u or anyone solve this and give me the answer with ofcourse correct method........

23
qwerty ·

wat i mean is , u cant apply analogy anywhere randomly, i hope u get wat i m trying to say

btw if u assumed something , then check whether the ans is within your assumption. i.e , u assumed y to be infintely large, but it actually came out to be 1-3log2 , so the final ans is contradictory to the assumption , so the assumption shud be wrng

23
qwerty ·

1-3log2 is not ∞

and 2 + (1-3log2 ) ≠1-3log2

u applied a concept , do u kno the basics of dat concept ?

1
nihal raj ·

i have used here the concept we apply in infinite serirs of resistors or capacitors...u remember.....

taking logarithm on an indefinite number will also be indefinite like 2+∞=∞ and much more....

1
sri 3 ·

@ Nihal

Let z = \sqrt{7+\sqrt{7+\sqrt{7+......infinity}}}

Now cover the first square root to get

\sqrt{7+\sqrt{7+......infinity}} which is also equal to z

(i.e.) \sqrt{7+\sqrt{7+......infinity}} = z

Therefore z =\sqrt{7+z}

Square both sides 2 get a quadratic in z

23
qwerty ·

@ nihal , why do u think that y is an infinite number ?

and also , if u assumed it to be an infinite number , did it turn out to be infinite ?

u said log7log7y= y

is ur answer y = 1-3log2 satisfying it ?

@ pritishmast i think wat u have done is correct if calc are correct

1
nihal raj ·

if acc. to u log7log7y=y is not possible then how can be - z=√z+7 possible in ur solution???

1
pritishmasti ............... ·

ans will be -0.265 only unless i haven''t made any calculation mistakes
@vivek
harry is correct
the complex looking number \sqrt{7+\sqrt{7+\sqrt{7+.........infinity}}} is not undefined or infinity .....it has a definite value.....calculating the value of this number
is just a simple 10th class problem.... only extra logarithm concept has been added

1
harrys ·

according to given solution ans will be same for every question like that
i.e. log7log7y=y
for any value of y which is an ∞ series
Ex.
y=">

1
nihal raj ·

can any one solve this????

1
harrys ·

how can you take log7log77+√7+√7....∞ =y
and also √7+√7+√7....∞ =y

1
nihal raj ·

solution....

let the infinite term be y.then

log7log7y

taking log on an indefinite i.e.,∞ no. gives infinite no. so,

log7log7y = y

differentiating both sides w.r.t. y gives

1log7y X log7e X 1y X log7e=1

log7logy X logelog7 X 1y X logelog7=1

1logy.log7.y=1

y.logy=1log7

again differentiating w.r.t. y

1 + logy =0

logy=-1

y=110

y=.1

or y=1-3log2

that is the answer.....

1
nihal raj ·

ur steps r right but method seems to be wrong............

1
nihal raj ·

answer is not -0.265

1
pritishmasti ............... ·

oh so it is like this:
\sqrt{7+\sqrt{7+\sqrt{7+\sqrt{7+\sqrt{7}}}}..............infinity }

let y=log7log7(z)

where z = \sqrt{7+\sqrt{7+\sqrt{7+\sqrt{7+\sqrt{7}}}}..............infinity }

or z = \sqrt{7+z}

solving this we get z= 1±√292

z=1-√292 neglected because
log7(1-√292 )=undefined as 1-√292 < 0

so z= 1+√292

so required value of y = log7log7(1+√292)

y = -0.265

1
nihal raj ·

hey, (7+......∞)≠(7+8+9+....∞)

dots signify that the series continues....

7+....∞={7+[7+(7+....∞)1/2]1/2}1/2

now solve it..........

1
nihal raj ·

hey akhilkona , bad luck....the answer is not 1....

1
pritishmasti ............... ·

is that 7+.....∞ =7 + 8 + 9 + 10........+ ∞ ???

donno why im getting answer ∞

6
AKHIL ·

is the ans 1???

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