if sinx=(4/5), where 0 degrees<x<90 degrees, find the value of cos (x+180)

25+25+100+n=250

since x is in the first quadrant, then x+180º is in the third quadrant, and cos(x+180) = =cos x

You should recognize the 3,4,5 right-angled triangle and cosx = 3/5

so cos(x+180)=-3/5

3x+y=90+2x+y=90

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