Write a function to find the largest triangle that can be inscribed in an ellipse.
import math
def largest_triangle(a,b): 
    if (a < 0 or b < 0): 
        return -1 
    area = (3 * math.sqrt(3) * pow(a, 2)) / (4 * b);  
    return area 
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find the largest triangle that can be inscribed in an ellipse.
import math
def largest_triangle(a,b): 
    if (a < 0 or b < 0): 
        return -1 
    area = (3 * math.sqrt(3) * pow(a, 2)) / (4 * b);  
    return area [/INST]assert largest_triangle(4,2)==10.392304845413264
assert largest_triangle(5,7)==4.639421805988064
assert largest_triangle(9,1)==105.2220865598093</s>