1.

If a triangle is inscribed in a n ellipse andtwo of its sides are parallel to the given straight lines, then prove thatthe third side touches the fixed ellipse.

Answer» Let the eccntric angls of the vertices P,Q, R of `DeltaPQR "be" theta_(1),theta_(2),theta_(3)`, respectively.
Then the equations of PQ and PR are, respectively,
`(x)/(a)cos.(theta_(1)+theta_(2))/(2)+(y)/(b) sin.(theta_(1)+theta_(2))/(2)=cos.(theta_(1)-theta_(2))/(2)`
`and (x)/(a)cos.(theta_(2)+theta_(3))/(2)+(y)/(b) sin.(theta_(2)+theta_(3))/(2)=cos.(theta_(2)-theta_(3))/(2)`
If PQ and PR are parallel to the given straigth lines, then we have
`theta_(1)+theta_(2)` (Constatn = `2alpha` (say)
and `theta_(1)+theta_(3)` = Constant= `2beta" "(1)`
Hence, `theta_(1)-theta_(3)= 2 (alpha-beta)`
Now, the equation of QR is
`(x)/(a)cos.(theta_(2)+theta_(3))/(2)+(y)/(b)sin.(theta_(2)+theta_(3))/(2)=cos.(theta_(2)-theta_(3))/(2)" "(2)`
or `(x)/(a)cos.(theta_(2)+theta_(3))/(2)+(y)/(b)sin.(theta_(2)+theta_(3))/(2)=cos(alpha-beta)" "(3)`
Which shows that line (2), for different values of `theta_(2)+theta_(3)` , is tangent to the ellipse
`(x^(2))/(a^(2))+(y^(2))/(b^(2))=cos^(2)(alpha-beta)`


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