
P(x1,y1) lies inside the triangle ⇒x1,y1∈N
x1+y1<41
∴2≤x1+y1≤40
Number of points inside = Number of solutions of the equation
x1+y1=n
2≤n≤40
x1≥1,y1≥1
(x1−1)+(y1−1)=(n−2)
(Number of non - negative integral solutions of x1+x2+.....+xn=r is n+r−1Cr)
Number of solutions =Cn−22+(n−2)−1
=n−1Cn−2
=n−1
We have 2≤n≤40
∴ Number of solutions =1+2+......+39
=239×40
=780