# Chaitanya's Random Pages

## January 31, 2012

### Recent fast test cricket centuries

Filed under: cricket,sport — ckrao @ 8:05 pm

David Warner recently scored 180 off 159 balls in a test cricket match against India, which ended up being the third fastest score of 180+ in terms of strike rate (runs/balls) (some older innings don’t have strike rate information). The only faster innings were the 222 scored off 168 balls by Nathan Astle vs England and the 293 off 254 by Virender Sehwag against Sri Lanka.

Michael Clarke scored his more recent 210 speedily too, with only 20 scores of 210+ having a faster strike rate (out of 210 other scores, some which have no data on balls faced). Clarke and Ponting posted a partnership of 386, which was Australia’s largest since 1946 and the 19th highest overall. Some records from day 2 of the 3rd test are here and here. It was the fourth time a triple century and double century were scored by a batsman in a series and the 15th time that two 200+ scores were posted by a batsman in a series (full list here).

Finally Clarke’s triple century in the 2nd test was the 25th overall and the 7th fastest in terms of strike rate (Hammond’s 336* was probably quicker but we don’t know how many balls he faced). He is one of very few players to average 45+ in both tests and ODIs.

## January 9, 2012

### Extremal points of a triangle

Filed under: mathematics — ckrao @ 11:41 am

In this post we summarise some of the points of a triangle that maximise or minimise various functions on the plane. This is on a similar theme to an earlier post on optimisation problems given a point inside a given angle. Many of the results are proved in [1].

Firstly we introduce some notation and terminology. Let $ABC$ be a given triangle with side lengths $a = BC,b = AC, c = AB$ and let $P$ be a variable point in the plane. Let $x, y, z$ be the distances from $P$ to the sides $AB, AC, BC$ respectively. The feet of the perpendiculars from $P$ to the sides of $ABC$ form a pedal triangle. If $AP, BP, CP$ meet sides $BC, CA, AB$ in points $D, E, F$, then $\bigtriangleup DEF$ is known as the Cevian triangle of $P$.

Functions involving distances to the vertices

• $\displaystyle \min |PA| + |PB| + |PC|$: $P$ is the Fermat point of $ABC$. If the largest angle is 120 degrees or more, the Fermat point is at the vertex of this angle, otherwise we can construct it by drawing an equilateral triangle $ABD$ external to $ABC$, then finding the intersection of $CD$ and the circumcircle of $ABD$.
In this figure one can show that $PA + PB + PC = CD$.
• $\displaystyle \max \min \{|PA|, |PB|, |PC|\}$: if $ABC$ is not obtuse, $P$ is the circumcentre of $ABC$. Otherwise it is the intersection of the longest side of $ABC$ with the perpendicular bisector of its middle-length side.
• $\displaystyle \min \{aPA + bPB + cPC\}$: $P$ is the orthocentre $H$ of $ABC$, with minimum value given by four times the area of $ABC$.
• $\displaystyle \min PA^2 + PB^2 + PC^2$: $P$ is the centroid $G$ of $ABC$ with minimum value given by $(a^2 + b^2 + c^2)/3$.
• $\displaystyle \min aPA^2 + bPB^2 + cPC^2$: $P$ is the incentre $I$ of $ABC$ with minimum value given by $abc$.
• $\displaystyle \min xPA^2 + yPB^2 + zPC^2$: This generalises the previous two expressions: $P$ is the point described by the position vector $(xA + yB + zC)/(x+y+z)$ with minimum value equal to $(a^2yz + b^2xz + c^2 xy)/(x+y+z)$.
• $\displaystyle \min PA.PB.c+ PB.PC.a + PC.PA.b$: $P$ is the orthocentre $H$ of $ABC$ with minimum value given by $abc$.

Functions involving distances to the sides

• $\displaystyle \min x + y + z$: $P$ is the vertex of the largest angle of $ABC$.
• $\displaystyle \max xyz$: $P$ is the centroid $G$ of $ABC$.
• $\displaystyle \max xy + yz + zx$: $P$ is the Mittenpunkt of $ABC$, given by the position vector $[a(b+c-a)A + b(c + a - b)B + c(a + b - c)C ]/[2(ab + bc + ca) - (a^2 + b^2 + c^2) ]$. This and the previous fact can be proved using Lagrange multipliers using the constraint that $ax + by + cz$ is constant (equal to twice the area of $ABC$). The Mittenpunkt is the point of intersection of lines joining the triangle’s excentres (formed by exterior angle bisectors) and midpoints.
• $\displaystyle \max \frac{xyz}{PA.PB.PC}$: $P$ is the incentre $I$ of $ABC$ (IMO shortlist 2001)

Functions involving the pedal triangle

• maximum area of pedal triangle: $P$ is the circumcentre $O$ of $ABC$.
• minimum perimeter of pedal triangle: $P$ is the orthocentre $O$ of $ABC$.
• minimum pedal radius (radius of circumcircle of pedal triangle): $P$ is the incentre of $ABC$. See [2] for more details.

Functions involving the Cevian triangle

• maximum area of Cevian triangle: $P$ is the centroid $G$ of $ABC$ with maximum area given by a fourth of the area of $ABC$.

References

[1] T. Andreescu, O. Mushkarov, L. Stoyanov, Geometric Problems on Maxima and Minima, Birkhäuser, 2006.

[2] V. Naik, Optimization methods in planar geometry, available at http://www.cmi.ac.in/~vipul/olymp_resources/olympiadarticles/geometricoptimization.pdf

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