Evaluate
3 2 + 5 + 3 2 − 5 .
Give your answer to 3 decimal places.
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.
How did 3(A + B) get transformed into 3AB?
Let y = 3 2 + 5 + 3 2 − 5 . Cubing it on both side, we get
y 3 = ( 3 2 + 5 ) 3 + ( 3 2 − 5 ) 3 + 3 ( 3 2 + 5 ) ( 3 2 − 5 ) ( 3 2 + 5 + 3 2 − 5 ) y 3 = ( 2 + 5 ) + ( 2 − 5 ) + 3 ( 3 ( 2 + 5 ) ( 2 − 5 ) ) ( y ) y 3 = 4 − 3 y y 3 + 3 y − 4 = 0
By trial and error method, we get y = 1 is a solution of this cubic equation. So, by synthetic division, we get
y 3 + 3 y − 4 = ( y − 1 ) ( y 2 + y + 4 )
So, y 3 + 3 y − 4 = 0 ⟹ y = 1 or y 2 + y + 4 = 0 . But the roots of y 2 + y + 4 = 0 are non-real complex numbers, because the discriminant of the quadratic is negative. The surd given to us is real. So, the only possibility is y = 1 .
Therefore, 3 2 + 5 + 3 2 − 5 = 1
Interesting approach with writing y as a polynomial. Is this approach work in the more general case (of roots)?
The same I did. An easy one
same thing, but insted of "trail and error method" use "rational root theorem"
How did u factorized the equation(y^3+3y-4) Pls :) ant method or something else ??
I've done the same :D
We note that: ( 1 ± 5 ) 3 ⇒ ( 2 1 ± 5 ) 3 = 1 ± 3 ( 1 2 ) 5 + 3 ( 1 ) ( 5 ) ± 5 5 = 1 6 ± 8 5 = 2 ± 5
Therefore, 3 2 + 5 + 3 2 − 5 = 3 ( 2 1 + 5 ) 3 + 3 ( 2 1 − 5 ) 3 = 2 1 + 5 + 2 1 − 5 = 1
How did you thought about ( 1 ± 5 ) 3 ?
Log in to reply
Initiation, I thought about what the problem setter was thinking when he set the problem. Problems of this type, we can usually try assuming 3 2 ± 5 = 3 ( a ± b 5 ) 3 = a ± b 5 so that the calculations become easy. So now we have:
2 + 5 = ( a ± b 5 ) 3 = a 3 + 3 a 2 b 5 + 1 5 a b 2 + 5 b 3 5
Equating coefficients:
{ a 3 + 1 5 a b 2 = 2 3 a 2 b + 5 b 3 = 1
Then you can solve for a and b .
I didn't do it that way. I just tried the simplest form ( 1 + 5 ) 3 = 1 + 3 5 + 1 5 + 5 5 = 1 6 + 8 5 , ⟹ a = b = 2 1 .
Log in to reply
Why did you assume 3 2 ± 5 = 3 ( a ± b 5 ) 3 and not 3 2 ± 5 = 3 ( a ± b c ) 3 ?
Log in to reply
@Aditya Sky – Then I will have one unknown less. Obviously, 5 is present and c = 5 . It will disappear only if b = 0 . Since LHS has 5 , the RHS must have it or else how could it come about. Note that 5 is a prime.
Problem Loading...
Note Loading...
Set Loading...
Let A = 3 2 + 5 , B = 3 2 − 5 .
And, ⇒ S = A + B = 3 2 + 5 + 3 2 − 5
Now, Cubing both sides.
⇒ S 3 = ( A + B ) 3
= A 3 + B 3 + 3 A B ( A + B )
= A 3 + B 3 + 3 A B S ...... ( ♧ ) [ Since , ( A + B ) = S ]
Thus,
⇒ A 3 + B 3 = ( 3 2 + 5 ) 3 + ( 3 2 − 5 ) 3
= 2 + 5 + 2 − 5
= 4
⇒ 3 A B = 3 ( 3 2 + 5 ) ( 3 2 − 5 )
= 3 ( 3 4 − 5 )
= 3 ( 3 − 1 )
= 3 × ( − 1 )
= − 3
Now, putting ( A 3 + B 3 ) = 4 and ( 3 A B ) = − 3 in ( ♧ ) .
⇒ S 3 = 4 − 3 S
S 3 + 3 S − 4 = 0
( S − 1 ) ( S 2 + S + 4 ) = 0
S − 1 = 0
S = 1
⇒ 3 2 + 5 + 3 2 − 5 = 1 .
Note : There is a possibilty that ( S 2 + S + 4 = 0 ) ,but this is not possible because its roots are non-real complex numbers.And the surds given is real.