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प्रश्न
Rekha has 15 square colour papers of sizes 10 cm,11 cm,12 cm, …, 24 cm. How much area can be decorated with these colour papers?
उत्तर
102 + 112 + 122 + … + 242
= (12 + 22 + … + 242) – (12 + 22 + … + 92)
= `(("n"("n" + 1)(2"n" + 1))/6)_(n = 24) - (("n"("n" + 1)(2"n" + 1))/6)_("n" = 9)`
= `(24 xx 25 xx 49)/6 - (9 xx 10 xx 19)/6`
= 4900 – 285
= 4615
∴ Rekha has 4615 cm2 colour papers. She can decorate 4615 cm2 area with these colour papers.
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