Gujarati
9.Straight Line
medium

The base $BC$ of a triangle $ABC$ is bisected at the point $(p, q)$ and the equations to the sides $AB$ and $AC$ are respectively $px+qy= 1$ and $qx + py = 1.$ Then the equation to the median through $A$ is

A

$(2pq - 1)(px + qy - 1) = ({p^2} + {q^2} - 1)(qx + py - 1)$

B

$({p^2} + {q^2} - 1)(px + qy - 1) = (2p - 1)(qx + py - 1)$

C

$(pq - 1)(px + qy - 1) = ({p^2} + {q^2} - 1)(qx + py - 1)$

D

None of these

Solution

(a) Since the median passes through $A$, the intersection of the given lines. Its equation is given by $(px + qy – 1) + \lambda (qx + py – 1) = 0$, where $\lambda $ is some real number. Also, since the median passes through the point $(p, q)$, we have $({p^2} + {q^2} – 1) + \lambda (qp + pq – 1) = 0$

==> $\lambda = – \frac{{{p^2} + {q^2} – 1}}{{2pq – 1}}$ and the equation of median through $A$ is $(px + qy – 1) – \frac{{{p^2} + {q^2} – 1}}{{2pq – 1}}(qx + py – 1) = 0$

==> $(2pq – 1)(px + qy – 1) = ({p^2} + {q^2} – 1)(qx + py – 1)$.

Standard 11
Mathematics

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