Euler is the first person who started to use radian in trigonometry. The circumference of a unit circle is

**2pi**, therefore he defined 360 degrees be**2pi**radian. A radian is the measure of an angle subtended at the centre of a circle by an arc equal in length to the radian.Why do they call it PI?

It was first

**called**"**pi**" in 1706 by [the Welsh mathematician] William Jones, because**pi**is the first letter in the Greek word perimitros, which means "perimeter." It's a special number.What is the name for half of pi?

March 14, or 3/14, is

**known as**“**Pi**Day” because of its resemblance to the first three digits in the decimal expansion of $**pi**$ (**pi**), which is defined as the ratio of a circle's circumference to its diameter: The true circle constant is the ratio of a circle's circumference to its radius, not to its diameter.