My letters play tricks on sight, R or A? A or R? Shuffle me and you will find, I am my own definition. What am I?

Two, four, six, eight, Even numbers sequence great! Zero, then one, one and two, Fibonacci’s are nice too. But this sequence that I mean Starts: one, three, six, ten, fifteen. Don’t feel like a silly goose, just Try a cute or an obtuse guess.

In this outdoor game, only two players may touch the ball with their hand. Also, some find one particular rule difficult to understand. I'm talking about the situation where a player is on the opponent's half and any part of them is nearer to the opponent's goal line than the ball or the second-to-last opponent.

Whether words, numbers, Or shapes are involved, I'm a challenge, a game, And a test to be solved.

Not on the A list, but I’m second best Use me to play tennis, not a ball, or a net In a sentence or maths . . .I square off the rest.