For, small values near 0, i.e., ∣x∣<<..1
y=k=1∑6kcos−1(coskx⋅cosα−sinkx⋅sinα)
=k=1∑6kcos−1(cos(kx+α))
=k=1∑6k(kx+α)
=k=1∑6(k2x+kα)
dxdy=k=1∑6k2=66(7)(13)=91
JEE Main 2020 — Mathematics Trigonometry
If y=k=1∑6kcos−153coskx−54sinkx then dxdy at x=0is
Held on 2 Sept 2020 · Verified 6 Jul 2026.
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