The sum of two angles in a triangle is equal to the third angle and two sides of the triangle have lengths of 4 and 5 units. Find the length of the third side of the triangle, if it is a whole number.

Angle1 + angle2+angle3=180 (property of triangle)

Angle1+angel2=angle3 (according to question)

Thus,

Angle3+angle3=180

2*angle3=180

Angle3=90

Means this is a right triangle

Now, we can use formula

Side1^2+side2^2=side3^2

Thus,

4*4+5*5=side3^2

Square root (36) = side3

Side3=6