Question : A rectangular LCD has an area of 480 cm2 and a perimeter of 92 cm. If the size of the LCD is defined in terms of the length of the diagonal, then what is the size of the LCD?
Option 1: 34 cm
Option 2: 39 cm
Option 3: 31 cm
Option 4: 35 cm
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Correct Answer: 34 cm
Solution :
The perimeter of a rectangular LCD is = 92 cm
Area is = 480 cm$^2$
Let the length = l and breadth = b
Then, 2(l + b) = 92
⇒ l + b = 46 … (1)
And lb = 480 ….(2)
Now (l – b)$^2$ = (l + b)$^2$ - 4lb
⇒ (l – b)$^2$ = 2116 - 4 × 480
⇒ (l – b)$^2$ = 2116 - 1920 = 196
⇒ l – b = 14 ….(3)
By adding, equations (1) and (3)
⇒ 2l = 60
⇒ l = 30 cm
⇒ b = 16 cm
Then, (Diagonal)$^2$ = 30
2
+ 16
2
⇒ (Diagonals)$^2$ = 1156
⇒ Diagonals = 34 cm
Hence, the correct answer is 34 cm.
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