Primary 6 Math?

K shared a pic he saw on fb, purportedly from a primary 6 math question:

Photo 8-1-14 11 27 25 pm

I saw the question when I just woke up and replied. It seem easy enough. However, after posting I immediately realized its wrong. The usual area subtraction tricks doesn’t seem to work. Well it seems obvious that it’s the area of 2 intersecting circles.

fig-1

The problem is I couldn’t recall learning any formula for it. I suspected that it involves geometry and trigonometry, but do they learn these things in primary 6? It bothered me enough to do a search and it turns out the following answer from Wolfram – which has this slightly threatening formula:

Equation 14

fig-2

Using the formula substituting r=4, R=8 and d=sqrt( 8*8 + 8*8 ), I got A = 1.727623528

Can someone verify if this is correct? Is there an easier way? Math teachers?

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