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Prove that 2? = 133° 34’, when the volume of concrete is minimum. Make any assumption...

Prove that 2? = 133° 34’, when the volume of concrete is minimum. Make any assumption where necessary.

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Answer #1

Consider an arch of

  • Thickness 't'
  • Angle - 2 ?
  • Radius - 'r'
  • Total length of the arch - 'L'
  • Area of the arch - A = r * Constant = kr

V= A *r * 2?

= k r *r *2?

=k *r​​​​​​2  *2?

Now ,

Sin ? = Perpendicular / Base

Perpendicular = (Total Lemgth of the arch /2) = L/2

Base = Radius = r

Sin ? = (L/2) / r

Sin? = L/2r

2r = L/Sin ?

r = L/2Sin?

Now ,

Volume = k * (L /2Sin ?)2 *2? =2? kL 2 / 4 Sin 2 ?

Differentiating both sides

dV/d? = d/d? (2?k L​​​​2  /4 Sin 2 ? )

For minimum volume of concrete dV/d? = 0

0 =( kL​​​2 / 2 ) d/d? (?/Sin​2 ? )

0 = (Sin 2 ? - 2?Sin?Cos?) / Sin 4 ? )

Sin 2 ?-2?Sin?Cos? = 0

Sin 2 ?=2?Sin?Cos?

Tan ? = 2?

133°34' = 2?

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