440. [K<sup>1</sup>. 7. a.] To find the condition that the four straight lines represented by ax<sup>4</sup>+4bx<sup>3</sup>y+6cx<sup>2</sup>y<sup>2</sup>+4dxy<sup>3</sup>+ey<sup>4</sup>=0 shall form a harmonic pencil
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