1 3. parabola. al Wites The cable o...
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1 3. parabola. al Wites The cable of a uniformly loaded suspension bridge hangs in the form of a The roadway which is horizontal and 100 m long is supported by vertical attached to the cable, the longest wire being 30 m and the shortest being Find the length of a supporting wire attached to the roadway 18 m from middle. 1g 6 m.

JEE/Engineering Exams
Maths
Solution
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Here AB is the longest wire (30m) and (OC) is the shortest at ( 6 mathrm{m} ). LN is the supporting wire, ( 18 mathrm{m} ) from the middle. We can see that ( B C= ) half of ( A B=50 mathrm{m} ) The shape of the parabola opens upwards, therefore the equation is ( x^{2}=4 a y ) We can see that coordinates of A is (50,24) (i.e, ( y ) coordinate is ( A B-O C=30-6=24 ) ) since ( A ) is a point on the parabola, ( 50^{2}=4 times a times 24^{2} rightarrow a=frac{625}{24} ) Therefore, the equation of the parabola is ( x^{2}=4 times frac{625}{24} y rightarrow 6 x^{2}=625 y ) ( x ) coordinate at point ( L=18 rightarrow 6 times 18^{2}=625 y rightarrow y ) coordinate at point ( L=frac{6 times 18^{2}}{625} approx 3.11 ) We need to calculate LN, which is ( 3.11+6=9.11 mathrm{m} ).
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