In a certain positive fraction b a , the numerator is 2 less than the denominator. If the numerator and denominator are interchanged, the fraction is increased by 1 1 5 1 . Find a + b .
Details and assumptions
A positive fraction is a fraction that has a positive value. For example, 1 2 1 0 and − 0 . 3 − 2 . 3 are both positive fractions.
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Why if 5 3 = b a , we must have a = 3 , b = 5 ? Why can't we have a = 6 , b = 1 0 or even a = 0 . 3 , b = 0 . 5 ?
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Calvin, bom dia! Note que se tivéssemos a referida fração, isto é 1 0 6 , o enunciado não seria satisfeito - "... o numerador é 2 menos que o denominador." Vale destacar que, se a diferença entre eles é 2, temos: ambos são pares, ou, ímpares! E, atendida tal condição, nossa resposta é de fato 3 e 5 .
a b − b a = 1 5 1 6
a b b 2 − a 2 = 1 5 1 6
a b ( b + a ) ( b − a ) = 1 5 1 6
a b = 1 5 a and b are either 1 and 15, or 3 and 5. Plugging 3 and 5 gets us this:
( 5 + 3 ) ( 5 − 3 ) = 1 6
8 ( 2 ) = 1 6
1 6 = 1 6
so a and b are 5 and 3. 5 + 3 = 8
This solution makes the invalid assumption that a and b are integers.
It seems to me that you made the assumption that a and b must be integers.
its easy...thank u...
Well, let a=value of numerator and let b=value of denominator. We know that the value of the numerator is 2 less than the value of the denominator so we can say a=b-2. So our original fraction which was a/b is now b-2/b. Next, this fraction has been interchanged which means that the numerator and denominator have been switched so our new fraction is b/b-2. It says that the original fraction has been increased by 1 1/15 which can also be written as 16/15. So our equation is b-2/b + 16/15 = b/b-2. By solving this equation we get the quadratic, -4b^2+23b-15. Using the quadratic formula, we get 2 roots which are 5 and 3/4. Since our answer has to be an integer, b is 5. Since the denominator is 5, the numerator is b-2 or 5-2 which is 3. Finally a+b is (b-2)+b; (5-2)+5; 3+5= 8 .
This is a good solution overall.
However, the statement "Since our answer has to be an integer, b is 5" shouldn't be used to reject the value of b = 4 3 . What other reasons (stated in the question) are there, which tells us that b = 4 3 ?
Value 3/4 will not make a positive fraction, as it will be -1.25/0.75
ahh
ahh
given that a=b-2, then you can make up an equation b/a= a/b +16/15 by the given statement above. then substitute the value of a, then it will be b/(b-2)=(b-2)/b + 16/15. then simplified it and you would get a value of 3/4 and 5. But since the value must be greater than 2 to come up with positive answer. Therefore the value is 5 and the value of a is 3. By just adding it you would get 8.
Can you explain what you mean by "Since the value must be greater than 2 to come up with positive answer"?
ya right
Let the den. be x. Therefore, num. is x-2. therefore, x-2/x - x/x-2 = 16/15 solving this eqn. , we get: x=-3,fraction = 5/3 therefore, 5+3 = 8
a/b =b/a-16/15 after solving 15a^2+16ab-15b^2=o given a=b-2 so after putting this value we got b=5 , a=3
a/b is positive fraction ,the numerator is 2 less than the denominator[a+2=b] lets a/b=x ,b/a=1/x ,If the numerator and denominator are interchanged, the fraction is increased by 16/15 [1/x=x+16/15] solve x=3/5 or x=-5/3[not suitable]because fraction is positive x=3/5 hence a=3b/5 and a+2=b solve that b=5,a=3 a+b=5+3=8
Let the numerator = a and denominator = b
Our fraction is a/b
The numerator is 2 less than denominator
So, a = b- 2
Our new fraction is b - 2/ b. Let's say the result is x.
Then
(b - 2)/ b = x -------------------------------------------------- ( i )
The next condition says that if the numerator and denominator are interchanged the result is 16 / 15 more than previous equation.
b / (b - 2) = x + ( 16/15 ) ---------------------------------------------- ( ii )
Substituting the value of x from equation ( i ) in equation ( ii ) we get,
b / (b - 2) = (b - 2)/ b + ( 16/15 )
or, (b / (b - 2)) - ((b - 2)/ b) = ( 16/15 )
Taking L.C.M. of the denominators as b ( b -2 )
((b ^ 2) - ((b - 2) ^ 2))/ (b ( b -2 )) = ( 16/15 )
Using the formula of ((a - b) ^ 2)) for ((b - 2) ^ 2)). We get
(b ^ 2) - (b ^ 2) + 4b + 4 / ((b^2) - 2b) = ( 16/15 )
4b + 4 / ((b^2) - 2b) = ( 16/15 )
By cross multiplication,
15 ( 4b - 4 ) = 16 (( b^2) - 2b)
60b - 60 = 16( b^2) - 32b
60b + 32b = 16( b^2) + 60
92b = 16( b^2) + 60
16( b^2) - 92b + 60 = 0
16( b^2) - 80b - 12b + 60 = 0
16b ( b - 5 ) - 12 ( b - 5) = 0
(16b - 12) (b - 5) = 0
Either, 16b - 12 = 0 _---------------------------------------( iii )
or, b - 5 = 0 -----------------------------------------------( iv )
From equation ( iii )
16b - 12 = 0
16b = 12
b = 12 / 16
b = 3 / 4
If we substitute value of b in the fraction then our fraction comes as -5/3 which is not our result. So, this value of b will not work.
Let's see another equation
From equation ( iv )
b - 5 = 0
b = 5
So, our denominator (b) is 5 and our numerator (a) is 3.
a + b = 8
denominator is 15 = 3 x 5 = (5 - 2) x 5 so a + b = 3 + 5 = 8
\frac{b}{b-2}=\frac{b-2}{b}+\frac{16}{15} \frac{b}{b-2}=\frac{15b-30+16b}{15b} Multiplicando cruzado temos 15b²=15b²-30b+16b²-30b+60-32b 16b²-92b+60=0-> Encontramos essa equação cujo conjunto solução é 5 e 3/4(Não convém). Logo temos que se b=5, e a =b-2 ,logo,a=3 e \boxed{a+b=8}
(x-2)/x=a/b…(1) x/(x-2)= a/b+16/15…(2) Substitute (1)—>(2) x/(x-2)=(x-2)/x+ 16/15 x/(x-2)= (31x-30)/15x 15x^2=31x^2-92x+60 0=16x^2-92x+60 make it simple, we will get : 0= 4x^2-23x+15 0=(4x-3)(x-5) x=3/4 or x=5 We need a positive solution, so we choose x=5 Substitute x=5 to the (1) we will find the fraction. That is 3/5 So a+b=3+5=8
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x − 2 x = x x − 2 + 1 5 1 6 1 5 x ⋅ x = 1 5 ( x − 2 ) ⋅ ( x − 2 ) + x ( x − 2 ) ⋅ 1 6 1 5 x 2 = 1 5 x 2 − 6 0 x + 6 0 + 1 6 x 2 − 3 2 x 1 6 x 2 − 9 2 x + 6 0 = 0 ÷ ( 4 4 x 2 − 2 3 x + 1 5 = 0 ( x − 5 ) ( x − 4 3 ) = 0 x − 5 = 0 x = 5
Daí,
x x − 2 = b a 5 5 − 2 = b a 5 3 = b a a + b = 3 + 5 a + b = 8