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**wopashui** Suppose that the random variables Y1 and Y2 have a joint probability distribution function f(y1, y2) given by

f(y1,y2)= $\displaystyle 6y1^2y2 $ 0<=y1<=y2, y1+y2<=2 and f(y1,y2)=0 elsewhere

(A) What is the probability that Y1 + Y2 is less than 1?

for this question, i have hard time findthe region of my integration, it seems like i need to use separate integral to do this, if we do it as dy2dy1 order, then y2 goes from y1 to 1-y1, but then y2 go from 0 to to separate function, so do i need to do it twice?