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805+ (Hard)|   Algebra|         
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when we factorize 945 we get - 3 cube*5*7
So number of factors are = (3+1)*(1+1)*(1+1) by formula
= 16
Ordered pairs therefore will be 16+16 = 32
Bunuel
If a and b are integers, for how many ordered pairs (a, b) is 945 = a^2 - b^2?

A. 8
B. 16
C. 24
D. 32
E. 64


 


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945 = 3^3 * 5*7
total factors =(3+1)*(5+1)*(7+1)=16

945=(a+b)*(a-b)... these factors can be maximum 16 numbers for positive a & b . For negative 16 more.

Total pairs = 32

Ans D
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If a and b are integers, for how many ordered pairs (a, b) is 945 = a^2 - b^2?
945= (a-b)(a+b)
3^3*5*7=(a-b)(a+b)
Number of positive divisors=(3+1)(1+1)(1+1)=4*2*2= 16.
Since 945 is odd, every factor pair is odd*odd, so all pairs are integers.
16 positive factor pairs gives 16 solutions and 16 negative factor pairs gives another 16.
Total ordered factor pairs =16+16=32

D
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(a+b)(a-b) = 945
prime factorization = 3^3 * 5 * 7
total number of factors = (3+1)(1+1)^2 = 16
as we have squares of integers, both positive and negative will work, so 16*2 = 32. Ans
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Bunuel
If a and b are integers, for how many ordered pairs (a, b) is 945 = a^2 - b^2?

A. 8
B. 16
C. 24
D. 32
E. 64


 


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945 = a^2 - b^2 = (a+b)(a-b)
On factorising we get 945 = 3^3 * 5 * 7
Hence the number of factors of 945 = (3+1)(1+1)(1+1) = 16

As 945 is the result of a difference of squares, we also consider negative numbers in the pair
Hence total pairs = 16*2 = 32 (D)
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Great question!

First find the number of factors of the 945 i.e we ll have 3^3 x 5 x 7 i.e 4 x 2 x 2 i.e 16 factors

Now 16 factors will create 8 pairs

Since pairs can be ordered, each paid can have 4 different values

i.e if the first pair is 945 = (a + b) * (a - b)

Then a + b = 945 and a - b = 1 we ll get 473 , 472

Now by changing signs we can have 4 pairs i.e (473, 472), (473 -472) , (-473, -472) and (-473, 472)

Hence 4 x 8 = 32

IMO option D


Bunuel
If a and b are integers, for how many ordered pairs (a, b) is 945 = a^2 - b^2?

A. 8
B. 16
C. 24
D. 32
E. 64


 


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(a+b)(a-b)= 945, given.
let (a+b) be X and (a-b) be Y,
XY=945

lets find the number of positive factors of 945= (3+1)(1+1)(1+1)= 16

16 is all the possible values of X, Y which are positive, but to count the negative values also we double 16= 32. thus there are 32 ordered pairs (a,b).

Bunuel
If a and b are integers, for how many ordered pairs (a, b) is 945 = a^2 - b^2?

A. 8
B. 16
C. 24
D. 32
E. 64


 


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We can expand a^2 - b^2 as (a+b)(a-b) = 945
If we factorize 945 into primes we get 3^3*5*7
Number of positive factors = (3+1)(1+1)(1+1) = 16 where (a+b),(a-b) both are +ve
So if both the numbers are negative then also we get 16 so total no of ordered pairs should be 16 + 16 = 32.
IMO Ans is D. 32.

Bunuel
If a and b are integers, for how many ordered pairs (a, b) is 945 = a^2 - b^2?

A. 8
B. 16
C. 24
D. 32
E. 64


 


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a^2-b^2 is basically a-b*a+b hence we need to get a and b values such that it is equal to 945 as said before. Now, 945=5*7*3^3 hence with the method of counting dordered pairs our next option is to count factors (3+1)(1+1)(1+1)= 16 and remember we need negative factors as well so 16+16=32. So D, is our answer.
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a^2-b^2 = 945
(a+b)(a-b) = 945

So, no.of ways = no. of factors of 945
945 = 3^3 * 5^ 1 * 7^1
Factors = (3+1)*(1+1)*(1+1) = 16
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Answer: B = 16

Given |a|>|b| since a^2 - b^2 is positive

Step 1: factor a and b

(a - b) (a + b)

Step 2: number of factors of 945

Prime factorization of 945 = 3^3 * 5^1 * 7^1

The number of factors in a number is the power of each prim factor plus one multiplied together.

(3+1) * (1+1) * (1+1) = 8

Step 3
This means there is 8 pairs of (a+b) and (a-b)

Since |a| > |b| there is 1 arrangement of a and b and they can be negative so every pair there is a negative counter part.

Thus 8 * 2 = 16
Bunuel
If a and b are integers, for how many ordered pairs (a, b) is 945 = a^2 - b^2?

A. 8
B. 16
C. 24
D. 32
E. 64


 


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a^2-b^2=945 indicates a^2>b^2. Thus, |a|>|b|.
For every combination of (a,b) that satisfies the equation, there will be four variants (a,b), (-a,-b). (-a,b), (a,-b) which also satisfy the equation.

We can find the total number of factors of 945 = (3+1)(1+1)(1+1) [as 945=(3^3)*5*7] = 16
Considering each of these 16 factors as a, we know for half of these scenarios - a will not be greater than b.

Therefore, we have eight possible combinations of (a,b) with four variants of each = 8*4 = 32
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First, since here a and b integars, i.e., it can be positive and negative. Moreover, as square can transform negative into positive, we will always get a positive number.

(a + b) x (a - b) = 945
(a + b) x (a - b) = 3 x 3 x 3 x 5 x 7
(a + b) x (a - b) = (3x3x5) x (3x7)
(a + b) x (a - b) = 45 x 21
Therefore, a + b = 45 and a - b = 21
Hence, 2a = 66, a = 33; 2b = 24, b = 12
Lets test, 33^2 = 1,089, 12^2 = 144. We get 1089-144 = 945

Therefore, we can make pairs: (33, 12), (-33, 12), (33, -12), (-33, -12), 4 pairs
945 has 3^3, 5^1, 7^1, (4x2x2) = 16 factors, the number of positive factors will be then, 16/2 = 8
Total ordered pairs = 8 x 4 = 32

Answer: D
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Given,
945 = a^2 + b^2
= (a + b)(a - b)

Let x = a - b, y = a + b

xy = 945
And, a = (x+y)/2 , b = (y - x)/2

So, x & y both must be either both odd or both even.

945 is odd, so, every pair of x & y are consist of two odd numbers. Also x & y must have same sign.


945 = 3^3 *5*7

Number of positive factors of 945 = (3+1)*(1+1)*(1+1) = 16
Total number of ordered factor pair = 16+ 16 = 32

Each pair value of x & y will correspond to the different pair of a & b .

Answer : D (32)

Bunuel
If a and b are integers, for how many ordered pairs (a, b) is 945 = a^2 - b^2?

A. 8
B. 16
C. 24
D. 32
E. 64


 


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945 = (a+b)*(a-b)=x*y
where a = (x+y)/2 and b = (x-y)/2
Here x and y are factors of 945.

945 = 3^3 * 5^1 * 7^1 = Total factors (Positive and Negative ordered) = (3+1)(1+1)(1+1) *2 = 32
Bunuel
If a and b are integers, for how many ordered pairs (a, b) is 945 = a^2 - b^2?

A. 8
B. 16
C. 24
D. 32
E. 64


 


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First I set 945 to equal (a-b)(a+b) based on difference of squares.

945 factors into 3*3*3*5*7, so the number of divisors equals (3+1)(1+1)(1+1) = 16 pairs for (a,b), the pairs can also be negative so that creates an additional 16 pairs.

16 positive pairs + 16 negative pairs equals 32 so the answer is D.
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a^2-b^2=945
(a-b)(a+b)=945
let a-b=x and a+b=y.
xy=945
945=3*3*3*5*7
No. of positive unique factors=(3+1)(1+1)(1+1)=16.
Since a and b are integers, x and y can be negative too.
so the no. of pairs are doubled.
So the answer is 32
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