Question 1159221
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I assume a is {{{ alpha }}} and b is {{{ beta }}}...

The trig functions take ANGLES as inputs, you can not plug in the output of other trig functions as those are RATIOS.   As such, you need to find angles a and b first:

a = {{{ tan^-1(-4/3) = 126.870^o }}}  (made use of "angle in Q2")
b = {{{ cot^-1(15/8) = 208.072^o }}}  (made use of "angle in Q3")

Now you can plug a and b into the equations:

1.  sin(a+b) = sin(126.870 + 208.072) = sin(334.942) = -0.424

etc.