The figure shows two waves traveling in the positive x direction. The amplitude of the resultant wave is

a. 2.0 mm
b. 1.8 mm
c.1.4 mm
d. 1.0 mm
e. 0.83 mm

Answer c.  1.4 mm

Explanation:

Notice that the addition of these two waves are equivalent to adding a sine wave plus a sine wave shifted by pi/2.

The maximum of such addition will come from twice the y value of the two functions at the point of intersect that takes place at about one grid unit to the right of the vertical axis. That intersection reads about 0.7 mm for the y value (which corresponds to an x of about pi/4)

Then, the addition of both y-values will give an amplitude of 1.4 mm

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