Problem: When the base of a triangle is increased 10% and the altitude to this base is decreased 10%, the change in area is:
Answer Choices:
A. 1% increase
B. 21​% increase
C. 0%
D. 21​% decrease
E. 1% decrease
Solution:
K0​=21​bh,Kn​=21​(b+.1b)(h−.1h)∴Kn​=21​bh+.05bh−.05bh−21​(.01)bh=(1−.01)K0​. Therefore, the change is a decrease of 1% of the original area.