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Algebra Level 3

Use the first 4 terms in the binomial expansion of 1 + 2 x \sqrt { 1+2x } to determine the value of 1.5 \sqrt { 1.5 } to 3 decimal places.

1.225 1.223 1.227 1.229

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1 solution

Geoff Taylor
Dec 30, 2017

Using a binomial expansion for 1 + 2 x o r ( 1 + 2 x ) 1 2 \sqrt { 1+2x } or\quad { (1+2x) }^{ \frac { 1 }{ 2 } }

The general binomial ( 1 + a ) n = 1 + n 1 ! a + n ( n 1 ) 2 ! a 2 + n ( n 1 ) ( n 2 ) 3 ! a 3 + . . . { (1+a) }^{ n }=1\ +\frac { n }{ 1! } a+\frac { n(n-1) }{ 2! } { a }^{ 2 }+\frac { n(n-1)(n-2) }{ 3! } { a }^{ 3 }+...

by letting a = 2x and n = 1/2

we obtain 1 + 1 2 ( 2 x ) + 1 2 ( 1 2 ) 2 ! ( 2 x ) 2 + 1 2 ( 1 2 ) ( 3 2 ) 3 ! ( 2 x ) 3 + . . . 1+\frac { 1 }{ 2 } \left( 2x \right) +\frac { \frac { 1 }{ 2 } \left( -\frac { 1 }{ 2 } \right) }{ 2! } { (2x) }^{ 2 }+\frac { \frac { 1 }{ 2 } \left( -\frac { 1 }{ 2 } \right) \left( -\frac { 3 }{ 2 } \right) }{ 3! } { \left( 2x \right) }^{ 3 }+...

Simplifying to 1 + x x 2 2 + x 3 2 + . . . 1+x-\frac { { x }^{ 2 } }{ 2 } +\frac { { x }^{ 3 } }{ 2 } +...

let x = 1 4 x=\frac { 1 }{ 4 } ... well within the convergence interval

We obtain x = 1.2267 rounding to 1.227

I was not looking for solvers to find the value by direct input into the calculator using the square root function.

So, we let x = 1 4 x = \frac{1}{4} but obtain x = 1.2267 x = 1.2267 ? Also, 1.5 1.22474 \sqrt{1.5} \approx 1.22474 ...

Will van Noordt - 3 years, 5 months ago

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the 1.22474 is directly obtain ... I was looking for solutions that involved the general expansion of the binomial theorem (1+x)^n

geoff taylor - 3 years, 5 months ago

I should have also told how many terms to use ... mea culpa ... srry ... I have adjusted

geoff taylor - 3 years, 5 months ago

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Makes sense now. Nice problem!

Will van Noordt - 3 years, 5 months ago

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