/* 後退代入 */
for( i = n-1; i >= 0; i-- ){
for( j = i+1; j < n; j++ ){
b[i] -= a[i][j] * b[j];
}
}

/* 出力 */
printf( "解A=%5.2f\n", b[0] );
printf( "解B=%5.2f\n", b[1] );
printf( "解C=%5.2f\n", b[2] );
printf( "解D=%5.2f\n", b[3] );

}

あと、氏ねカス