Latest Elitmus Aptitude Question SOLUTION: Find the value of a3 + b3. 1. a2+b2 = 13 , a+b=6 2. ab=20 a+b =9 Option: A.If one alone can give the answer but other can not give answer. B.If both together can give the answer.

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(#M40038291) ELITMUS Logical Reasoning question Data Sufficiency asked on 10/08/14 Keep an EYE

Find the value of a3 + b3.
1. a2+b2 = 13 , a+b=6
2. ab=20 a+b =9

Option:
A.If one alone can give the answer but other can not give answer.
B.If both together can give the answer.
C.If either of two can give the answer
D.Neither of two can give the answer.

Asked In Elitmus Rahul Bhaskar (9 years ago)

Unsolved Read Solution (13)

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Read Solution (13) :

Victor Saha  (9 years ago)

C. Either of the two can give the answer. (But ya the answers are different though).

Using 1)
a^2+b^2=13
Now, a+b=6
or, a^2 + b^2 +2ab = 36
or, 13 + 2ab = 36
or, ab=23/2

Thus, a^3 + b^3 = (a+b)(a^2 - ab + b^2) = 9

Using 2)
(a+b)^2=81
or, a^2 + b^2 + 2(20)=81
we get, a^2 + b^2 = 41

Thus, a^3+b^3=(a+b)(a^2 - ab + b^2)= 189.

Is this solution Helpfull? Yes (51) | No   

vartika   (9 years ago)

C... coz a3+b3= (a+b)(a2+b2+2ab)
from 1 and 2 both we can get all the reqd values , using : (a+b)^2= a2+b2+2ab, ie value of ab can be obtained using this identity

Is this solution Helpfull? Yes (8) | No (6)  

Pranoy Bej  (9 years ago)

ANSWER==> A
because 1 has no solution
for 2 we find a=4 , b=5 or vice-versa

Is this solution Helpfull? Yes (4) | No (16)  

Kumar Vikash  (9 years ago)

a3+b3=(a+b)3-3ab(a+b)

Is this solution Helpfull? Yes (3) | No (1)  

ASHUTOSH KUMAR  (9 years ago)

ans is (B)because the a3+b3 formula needs to all the above value.

Is this solution Helpfull? Yes (2) | No (10)  

abcxyz  (9 years ago)

Both of the questions can give the answer. We have the formula a^3-b^3 = (a+b)(a^2+b^2-ab) we can calculate it's value separately by both the statements.so answer is C.


@PRANOY BEJ- your answer is wrong as in cube roots of one x^3-1=0 we have roots 1,omega,omega^2, but multiplication of omega && omega^2 =1. In the same way as u are solving may be roots are imaginry but there product can be calculated :)

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ankit shw  (9 years ago)

C
a^3+b^3=(a+b)(a^2+ab+b^2)
using 1
ab=((a+b)^2-(a^2+b^2)}/2;
using 2
a^3+b^3=(a+b)[(a+b)^2-ab]

Is this solution Helpfull? Yes (2) | No   

rishabh raj  (9 years ago)

answer will be C....both statement can give the values..answers are different coz values of a+b are different in both the statements

Is this solution Helpfull? Yes (2) | No (1)  

BRAJESH YADAV  (9 years ago)

Answer will be option c because using either statements we can directly find the value of a and be.

Is this solution Helpfull? Yes (2) | No   

SUDHAKAR SRIVASTAVA  (9 years ago)

Either of them can give the answer.
a^3+b3=(a+b)(a^2 +b2 -ab)
..put the values from any of them in the eqn

Is this solution Helpfull? Yes (1) | No   

euro  (9 years ago)

Use formula: a3 + b3 = (a + b)(a2 – ab + b2)
we have given, 1. a2 + b2= 13 , a+b= 6 and also in 2. ab = 20, a+b=9
From the first we have found ab = 23/2
& from the second we can find a2 + b2 = 41
Now put in the formula we get it as 189 :)

Is this solution Helpfull? Yes (1) | No   

kavya chithra  (9 years ago)

how can the ans be C,we get different solutions for different options .so that if one alone can give answer the other cannot .Thus it should be A

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Digesh Singh Bisht  (1 year ago)

Both of the questions can give the answer.

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