GhostShooter

not rider

:)

and then proceed as spidey has done

1)

2)

image is at 2f

for convex lens

image distance =object distance

so magnification=1

so image height=object size=10cm

Select a point in ⁿC₁=n ways

now from each point we cannot draw a diagonal to 3 points... 2 points adjacent to it and the point itself

so n-3 such diagonals r there

also..each diagonal has 2 endings so it is counted twice

so D=n(n-3)/2

here variable is x^2

so it is same as integral of

sqrt{1+t^2}

it should be same as the combined equation of the lines bisecting the angle between given pair of lines