<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>