Step-by-step explanation:
A+B+C= 180
A+B= 180-C
cos(A+B)= cos(180-C)
cos(A+B)= -cosC [180-c belongs to 2nd quadrant]
#666
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Step-by-step explanation:
A+B+C= 180
A+B= 180-C
cos(A+B)= cos(180-C)
cos(A+B)= -cosC [180-c belongs to 2nd quadrant]
#666