Two sides of a triangle are 6 cm and 10 cm, and the angle between them is 150°. The area of the triangle is:

Area = (1/2)·b·c·sinA = $(1/2)(6)(10)$·sin150°. The sine of an obtuse angle is the sine of its supplement, so sin150° = sin30° = 1/2 and the area is $30\times 1/2=15{\text{cm}}^2$. Reaching for the cosine instead (|cos150°| = √3/2) gives 15√3 ≈ 25.98, and forgetting the factor 1/2 gives 30.