urgent answer please
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Answer:
=>cos^-1{1/√2 cos 9pi/10 - 1/√2 sin 9pi/10}
=>cos^-1{cos pi/4 cos 9pi/10- 1/√2 sin 9pi/10}
=>cos^-1{cos(pi/4+9pi/10)} ----------->cos(A+B)= cosA cosB - sinA sinB
=>cos^-1{cos(23pi/20)}
=>cos^-1{cos(pi+3pi/20)}
=>cos^-1{-cos(3pi/20)}
=>pi- cos^-1{cos(3pi/20)} --------------->cos^-1(-x) = pi- cos^-1(x)
=>pi-3pi/20= 17pi/20
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