If sin x = .21,what is cos x?

Let's try a fundamental identity.

sin(X)^2 + cos(X)^2 = 1  (0.21^2 =0 .0441)

cos(X)^2 = 1 -0.0441

cos(X)^2 = 0.9559

co(X) = sqrt(0.9559)

cos(X) = 0.98

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