Question : If $a, b,$ and $c$ are positive integers such that $a^2 + b^2 = 82$ and $b^2 + c^2 = 65$, then the value of $2a + 7b - 3c$ is:
Option 1: 2
Option 2: 5
Option 3: 49
Option 4: 1
Correct Answer: 1
Solution :
$a^2 + b^2 = 82$ --------------(1)
$b^2 + c^2 = 65$ ---------------(2)
Subtracting equation (2) from equation (1), we get,
$a^2-c^2=17$
⇒ $(a+c)(a-c)=17×1$
⇒ $a+c=17$ and $a-c=1$ [since $a, b$ and $c$ are positive integers]
⇒ $a=9$ and $c=8$
From equation (1), we get,
$9^2+b^2=82$
⇒ $b^2=1$
⇒ $b=1$
Now, $2a + 7b - 3c$
= $(2×9)+(7×1)-(3×8)$
= $18+7-24$
= $1$
Hence, the correct answer is 1.
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