<canvas width = "1200" height = "800"></canvas>
<script>
var tela = document.querySelector("canvas");
var pincel = tela.getContext("2d");
pincel.fillStyle = "green";
pincel.fillRect(0, 0, 1200, 800);
pincel.fillStyle = "yellow";
pincel.beginPath();
pincel.moveTo(600, 0);
pincel.lineTo(0, 400);
pincel.lineTo(1200, 400);
pincel.fill();
pincel.beginPath();
pincel.moveTo(600, 800);
pincel.lineTo(0, 400);
pincel.lineTo(1200, 400);
pincel.fill();
pincel.fillStyle = "darkblue";
pincel.beginPath();
pincel.arc(600, 400, 250, 0, 2*3.14);
pincel.fill();
</script>