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```
If x cos a + y sin a = p touches the cuve x^m .y^n = a^(m+n) ;prove that p^(m+n) . m . n = (m+n)^(m+n) cos^m a sin^n a

```
4 years ago Nishant Vora
IIT Patna
2467 Points
```							Simplify algebra by making subs X=x/a, Y=y/b and m=n/(n-1)Xᵐ + Yᵐ = 1 … (i)Xacotα + Yb = pcosecα … (ii)Diff (i) wrt X : mXᵐ⁻¹ + mYᵐ⁻¹(dY/dX) = 0 ⟹ dY/dX = −(X/Y)ᵐ⁻¹At pt of contact (X,Y) have grad (i) = grad (ii) so −acotα/b=−(X/Y)ᵐ⁻¹ … (iii)Sub in (ii) for acotα : bXᵐ/Yᵐ⁻¹ + Yb = pcosecα ⟹ b(Xᵐ+Yᵐ) = pYᵐ⁻¹cosecα ⟹ Yᵐ⁻¹ = (b/p)sinαAlso using (iii), Xᵐ⁻¹ = (a/p)cosαSub in (i) for X,Y and adjust to get (acosα/p)^(m/(m-1)) + (bsinα/p)^(m/(m-1)) = 1Setting m/(m-1)=n and multiply by pⁿ gives desired result
```
3 years ago
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